Oven baked fish casserole

with romesco sauce, tomato, and garlic

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Soft oven baked fish combined with a delicious homemade romesco sauce and garnished with tomato, red onion, fresh rosemary, and bay leaves.

This amazing summer casserole is super easy to make and can be whipped up in 30 minutes!

Easy Recipe | Casserole | Seafood | Mediterranean Recipe | Mediterranean | Fish Recipe | Easy Main Meal | Healthy Recipe | 30-Minute Recipe | Romesco Sauce Recipe

Serving:

Mains for 4

Ready in:

30 minutes

Skill level:

Easy

Serve with:

Bread, rice, or potatoes

Oven baked fish casserole ingredients sits on a grey granite kitcchen worktop

Ingredients you’ll need

Serves 4 as a Main

    For the casserole

    • 1kg/35oz. fresh white fish, heads removed and cleaned (or around 600g/21oz. of fish  fillets)
    • 1 red onion
    • 2 large tomatoes (or cherry tomatoes work well too)
    • 2 tablespoons of extra-virgin olive oil
    • 10 garlic cloves
    • A few springs of fresh rosemary
    • 1 teaspoon of Cumin seeds (optional)
    • 2 tablespoons of blanched almonds (or flakes) 
    • 10 bay leaves
    • 2 tablespoons of flat-leaf parsley (optional for garnish)

    For the romesco sauce

    • 3 tomatoes, grated and skin discarded
    • 200g/7 oz. blanched almonds
    • 4 cloves of garlic
    • 200g/7 oz. roasted peppers (if from the jar, drained)
    • 100ml/3fl. oz. olive oil 
    • 50ml/1.5fl. oz. sherry vinegar (or balsamic vinegar)
    • 1 tablespoon of smoked paprika
    • Salt and pepper for seasoning

          Equipment needed

          • A long sharp filleting knife (for filleting the fish)
          • A large 12-inch/30cm casserole dish with sides at least 2 inches/5cm tall. 
          • A blender or food processor for making the romesco sauce
          A large casserole dish sits with fish, tomato, red onion, and romesco sauce

          White fish recipes for summer

          One of the beauties of summer is the abundance of white fish that fills the counters of any reputable fishmonger, not to mention the amazing white fish recipes that conjure memories of hazy summer days and warm summer nights. Whether it be the aroma of the sea or the aroma of fried or baked fish dishes, white fish signals the very best of summer vibes.  

          White fish recipes are perfect for any time of year but seem to fit harmoniously with our tastebuds come summertime. White fish works so well with a diverse range of summer-infused ingredients. From crispy fried fillets with a tangy aioli and fresh wedges of lime, to white fish fillets that are baked to perfection with a little thyme, lemon, or rosemary. Some other awesome fish recipes for summer are Maravilla fish stew with chickpeas and pearl pasta, oven-baked cod with asparagus and cherry tomatoes, boquerones (cold tapas), and cod ceviche with pink grapefruit and pickled cucumber. 

          Fish seasoning

          If you’re frying fish, season white fish fillets lightly with salt and a little cracked black pepper. Perhaps a squeeze of lemon or lime and you’re good to go. Add a little spice with a pinch of cayenne or some ground red chili flakes. 

          When baking white fish, the world is your oyster. Use smoked paprika with a tomato or romesco-based sauce. Some fresh rosemary sprigs work particularly well with this combination as we’ve shown in this recipe.

          If you’re looking for a more tangy flavor, some ground cumin, lime, and cilantro works a treat as a marinade that can be drizzled over a whole fish. Mix it with some extra virgin olive oil and season lightly with some sea salt and cracked black pepper. 

          Romesco sauce and white fish

          Unsurprisingly, romesco and white fish fillets are a match made in heaven. Of course, this may have something to do with the fact that romesco sauce is a recipe concocted by Spanish fishermen. And if you love shrimp (prawns), then you have to try this spicy shrimp with romesco sauce tapas recipe.

          The story of the creation of romesco sauce originated from Valls, a small town in the province of Tarragona, Catalonia. Valls is famous not only for creating romesco, but also calçots (a type of green onion that is ceremoniously barbecued to mark the start of warmer weather after winter.)

          Homemade Romesco Sauce 

          Never buy romesco sauce again with this super-easy 10-minute romesco sauce recipe. There’s virtually no cooking required and you can dial up the amount of spice to cater to your specific recipe needs!

          You can also try romesco sauce with roasted chicken breast or make Mediterranean chickpea fritters with romesco as a dipping sauce. 

          A small bowl of romesco sauce sits with a jar full of romesco sauce in the background

          White fish types

          Here along the Mediterranean Sea, we are very lucky to have some excellent choices when it comes to fresh fish and seafood. Whenever I visit our local fishmonger, I go with an open mind and simply choose the fish that looks the freshest and best quality. 

          Some popular white fish types include;

          • Cod
          • Bass
          • Hake
          • Grouper
          • Haddock
          • Whiting
          • Catfish
          • Snapper

          The day I made this recipe, I was greeted with some excellent options. Ray fillets were finally back and had a wonderful opaque white color and good texture. Whole Lubina (Bass) were studiously lined up on a bed of ice, their scales glistening with a deep blue color. However, the fish that really caught my eye was the fresh blue whiting, called Bacaladilla in Spain. 

          Bacaladillas are usually around the same size as sardines commonly found here (about 6 inches/15cm from nose to tail). They have a more subtle flavor compared to sardines which makes them ideal for baking in the oven, especially with a rich sauce like romesco sauce.

          A few small blue whiting. sit on a plate waiting to. be filleted

          Best white fish recipes

          White fish is great for baking as it’s usually available year-round and is quite cheap. When cooking with white fish, it’s best to buy it fresh from a fishmonger and they will be able to offer you the freshest catches for the day. 

          For this recipe, we used Spanish blue whiting with very small fillets that came out at around 2-inches/5 cm wide, and around 4-5 inches/10-13 cm long. They’re only around 1/4-inch (1/2 cm) thick too so require very little time in the oven to cook. 

          White fish cooking tips for this recipe

          We have also made this recipe using larger fillets from some cod and we just needed to add an extra 8 minutes of cooking time to ensure they were cooked. As a rule of thumb, adjust the cooking time for fillets that are thicker than 1 inch/2.5 cm by adding 5-8 minutes (depending on your oven). 

          Is frozen white fish fillets good for baking?

          If you’re on a budget or can’t get fresh fish, there are some great frozen white fish fillets available from any large supermarket. When using frozen fish, ensure you give it time to thaw properly and remove any excess liquid by patting down each fillet with some paper towel. Also, take note that you may require a little seasoning with some salt to give the fish a bit more flavor.

          Some fish fillets, sliced red onion, and romesco sauce sit in a casserole dish waiting to be mixed.

          How to bake fish

          When baking whole fish in oven, it’s often best to use the middle shelf of the oven and use a tray that allows for the best air circulation. This will give the fish the best chance of cooking evenly. 

          Here are some guides on cooking times for whole fish. 

          • A whole fish weighing around 500g/ 17 oz. will take approximately 15 minutes to cook at 180°C/ 350°F on the middle shelf of the oven. Larger fish will take more time. 
          • A 12-inch/30 cm whole cod that is around 3-inches/7.5 cm thick will take approximately 20 minutes in the oven at 180°C/ 350°F. 
          • Fish thicker than 4 inches/10 cm will likely require at least 30 minutes in the oven at 180°C/ 350°F. 

          Baking fish fillets in the oven

          Pay attention to the thickness of the fillets and your baking times. A whole fish will take longer than fillets so adjust your cooking time accordingly. 

          Make sure you keep the temperature quite low (around 160-180°C /320- 350°F) and place the oven tray in the lower portion of the oven. 

          Most smaller white fish fillets that are under ½ inch/1.25 cm thick will only take around 15 minutes to cook at 180°C/ 350°F. White fish fillets that tare over 1-inch/2.5 cm thick will take around 20-25 minutes. 

          The small blue whiting fillets needed just 15 minutes to cook at 180°C/ 350°F along with the other ingredients in the casserole.

          A tray of fish is marinated in romesco sauce and garnished with. rosemary and bay leaves.
          oven baked fish casserole sits on a grey granite kitchen worktop beside some fresh sprigs of parsley.
          baked-white-fish-casserole-hero-square-v2-1080x

          Oven baked fish casserole in romesco sauce and garlic

          Soft oven baked fish combined with a delicious homemade romesco sauce and garnished with tomato, red onion, and fresh rosemary. Made in 30 minutes!
          5 from 9 votes
          Print Recipe Pin Recipe
          Prep Time 15 minutes
          Cook Time 15 minutes
          Total Time 30 minutes
          Course Main Course, main meal, seafood
          Cuisine American, Authentic Spanish recipe, Mediterranean, Seafood, Spain, spanish, summer
          Servings 4 mains
          Calories 708 kcal

          Equipment

          • 1 Long sharp filleting knife (for filleting the fish)
          • 1 Large 12-inch/30cm casserole dish with sides at least 2 inches/5cm tall.
          • 1 Blender or food processor for making the romesco sauce

          Ingredients

          For the casserole

          • 1 kg/35oz. fresh white fish heads removed and cleaned (or around 600g/21oz. of fish fillets)
          • 1 red onion
          • 2 large tomatoes or cherry tomatoes work well too
          • 2 tablespoons of extra-virgin olive oil
          • 10 garlic cloves
          • A few springs of fresh rosemary
          • 1 teaspoon of Cumin seeds optional
          • 2 tablespoons of blanched almonds or flakes
          • 10 bay leaves
          • 2 tablespoons of flat-leaf parsley optional for garnish

          For the romesco sauce

          • 3 to matoes grated and skin discarded
          • 200 g/7 oz. blanched almonds
          • 4 cloves of garlic
          • 200 r/7 oz. roasted peppers From the jar, drained
          • 100 ml olive oil
          • 50 ml sherry vinegar or balsamic vinegar
          • 1 tablespoon of smoked paprika
          • Salt and pepper for seasoning

          Instructions
           

          Step 1 - Prepare romesco sauce

          • Note: this step can be done in advance and stored in an airtight container or jar in the fridge.
            A small bowl of romesco sauce sits with a jar full of romesco sauce in the background
          • In a dry pan, lightly toast the blanched almonds on medium-high for 5 minutes. Stir well to stop them from burning. Allow almonds to cool before moving on to the next step.
            200 g/7 oz. blanched almonds
            toasted almonds in a large frying pan
          • Add the roasted red peppers, grated tomato, garlic, vinegar, and smoked paprika into a blender or food processor along with the toasted almonds.
            1 red onion, 4 cloves of garlic, 200 r/7 oz. roasted peppers, 50 ml sherry vinegar, 1 tablespoon of smoked paprika, 3 to matoes
            ingredients for romesco sauce sits in. a blender
          • Buzz for 3-4 minutes or until smooth.
            Blended romesco sauce in a blender
          • Slowly drizzle in the olive oil to achieve the desired consistency. A good texture should slowly drip from a spoon.
            100 ml olive oil
            Olive oil is poured into a small clear glass
          • Season with some salt and pepper to taste. Add any spice with some crushed chili or cayenne pepper as required.
            A small bowl of romesco sauce sits with a jar full of romesco sauce in the background

          Step 2 - Fillet and prepare fish

          • Preheat the oven to 180°C/ 350°F.
            Lightly grease a large casserole dish and set it aside for use later.
            An oven dial turned up to 180C
          • Using a sharp filleting knife, carefully fillet all of the fish and make sure all bones are removed (your fishmonger may be able to do this for you if you ask nicely!). Once filleted, clean the fillets in some fresh water and pat dry with some paper towel. Set aside for use later.
            1 kg/35oz. fresh white fish
            A few small blue whiting. sit on a plate waiting to. be filleted

          Step 3 - Mix sauce and other ingredients

          • Slice red onion super fine. Slice tomatoes into ½-inch slices.
            Add fish fillets, red onion, sliced tomatoes, and around 200ml/7fl.oz. of romesco sauce.
            1 red onion, 2 large tomatoes
            Some fish fillets, sliced red onion, and romesco sauce sit in a casserole dish waiting to be mixed.
          • Mix all ingredients well and spread out evenly in the casserole dish.
            a large casserole dish with some fish fillets, tomatoes, red onion, and romesco sauce mixed together.

          Step 4 - Add garlic and garnishing

          • Arrange peeled garlic, bay leaves, cumin seeds, and sprigs of rosemary around the casserole.
            10 garlic cloves, 1 teaspoon of Cumin seeds, 10 bay leaves, A few springs of fresh rosemary
            A tray of fish is marinated in romesco sauce and garnished with. rosemary and bay leaves.
          • Drizzle with some extra virgin olive oil.
            2 tablespoons of extra-virgin olive oil, 2 tablespoons of blanched almonds, 2 tablespoons of flat-leaf parsley, Salt and pepper for seasoning
            Olive oil is poured into a small clear glass
          • Season with some coarse sea salt and freshly cracked black pepper. Sprinkle with a few almond flakes and bake for 15-20 minutes at 180°C/ 350°F.
            A tray of fish is marinated in romesco sauce and garnished with. rosemary and bay leaves.

          Step 5 - Serve

          • Allow 5 minutes to cool before serving. Serve in the casserole dish with some fresh bread.
            A large casserole dish sits with fish, tomato, red onion, and romesco sauce

          Video

          Nutrition

          Serving: 200gCalories: 708kcalCarbohydrates: 60.5gProtein: 28.6gFat: 42.8gSaturated Fat: 5.9gSodium: 88mgPotassium: 639mgFiber: 7.2gSugar: 23.2gCalcium: 137mgIron: 3mg
          Keyword Casserole, Easy Dinner ideas, easy recipe, fish, healthy, mediterranean cuisine, oven roasted, Romesco Sauce, seafood, summer

          Oven-Baked Fish Casserole -Cooking Tips and FAQs

          What is the easiest way to fillet a fish?

          • Lay the fish out on a chopping board and ensure you are using a very sharp filleting knife. 
          • Hold the fish by the tail, and use the knife to cut the fillet from the tail to the head. 
          • Use a smooth motion and try to run the knife along the backbone of the fish. 
          • Carefully cut off the head and then remove the rib bones from the fillet. 
          • Trim off the fins and any excess fat or skin.
          • Finally, clean the fillets in fresh water and dry them on a paper towel. 

          How long can you cook a fish in the oven?

          It all depends on the size of the fish. Fish cooking guide below.  

          • A whole fish weighing around 500g/ 17 oz. will take approximately 15 minutes to cook at 180°C/ 350°F on the middle shelf of the oven. Larger fish will take more time. 
          • A 12-inch whole cod that is around 3-inches thick will take approximately 20 minutes in the oven at 180°C/ 350°F. 
          • Fish thicker than 4 inches will likely require at least 30 minutes in the oven at 180°C/ 350°F. 

          What temperature should I bake my fish casserole?

          If you are using small thin fish fillets like whiting, then bake your fish casserole at 180°C/ 350°F for 15 minutes. If your fish fillets are thicker, allow more time in the oven. 

          Is it better to bake fish covered or uncovered?

          As this recipe uses a thick sauce, you do not need to cover the fish. 

          A fish that is covered with foil will effectively ‘steam’ and this is desirable when you don’t want your fish to dry out.