Today’s recipe is a savory fish casserole made with caramelized onions, Swiss cheese, bread crumbs and white wine. This is a favorite family recipe of mine and works with any white flaky fish you like.

side view of the casserole with breadcrumbs and cheese on top

I originally shared this recipe on Healthy Seasonal Recipes on February 20th, 2015. I have updated the photos, added a video and updated the text to make this post more useful for you today. 

This happens to be one of my favorite fish recipes of all time. It is based on a recipe my grandmother invented when my uncle was in the habit of bringing home bass from his daily fishing trips. She served it to her family every summer, and my mom always made it for my family when I was a child. It can be made with any kind of white fish and it is both comforting and gourmet tasting at the same time. 


Ingredients For This Fish Casserole Recipe

Softened unsalted butter

To prevent the casserole from sticking to the baking dish, I greased it with softened butter. I used a generous amount because I like the flavor the butter adds to the recipe. 

White fish fillet

  • Today I used frozen thawed cod. To learn more about how to buy sustainable cod, check out my post and recipe for Cod Fish Tacos.
  • Feel free to use another white fish, but make sure to check for the Best Choices in your area when you shop.
  • If frozen, thaw in the fridge overnight or place sealed filets in a water bath for one hour to thaw quickly. If using the water bath method, make sure to use immediately.

Salt and Pepper

To season the recipe I added salt and pepper directly to the fish filets themselves. If you are on a sodium restricted diet, skip the salt as the bread crumbs and cheese will add enough salt for a palate that is accustomed to lower sodium levels. Do not sub in kosher salt for regular table salt until you read this post. 

Extra-virgin olive oil

To cook the onions and to moisten the breadcrumbs I used olive oil. You can also use a neutral oil such as avocado instead if you prefer. 

Sweet onion

The onions in this recipe are so amazing! The sweet caramelized flavor is incredible. 

Dry thyme and Nutmeg

You can use either dry thyme or chopped fresh thyme. Use half the amount if using dry as it is more concentrated. The nutmeg is a nice little touch that I like to add to savory dishes with cheese.

White wine

Choose a dry white wine such as pinot grigio, Gavi or Pinot blanc for this recipe. The dry (sour) taste is a great foil for the salty cheese and sweet onions. 


For the breadcrumbs I use fresh bread made into breadcrumbs. I have been making homemade sourdough for the last year so I have a loaf on hand all the time. You can also use multigrain if you happen to have that on hand. If it is more firm or stale it will chop in the food processor better. 

Paprika and Garlic powder

To season the breadcrumbs I added in paprika and garlic powder (which I do in breadcrumb mixtures all the time here.) It helps to add color and flavor to the topping and makes it appear more browned and decadent even with only a little fat added. 

Emmentaler, Gruyere or Swiss cheese

I know there is an Italian somewhere who is crying into his espresso because I have a fish recipe, but I am telling you that the cheese is what makes this recipe so special. Combined with the wine the flavor is similar to that of fondue! If you can find it look for Gruyere cheese, but Swiss or Emmentaler are also good alternatives and what my grandmother used for her version of this casserole. 

Baked Fish topped with breadcrumbs and cheese on a spatula

How To Make This Recipe

Before You Begin

Preheat the oven to 400 degrees. I also recommend buttering your baking dish too so that it is ready to go once your ingredients are ready to add to the casserole.

Casserole Dish

Choose a casserole dish that your portions of fish will fit preferably without overlapping. I used a 9 by 13 inch baking dish, but an oval or another shape is fine as long as they fit without crowding. Note: if your fish filets are thin like catfish, tilapia, sole or haddock it is okay to overlap them up to 1 inch thickness.  

Make Caramelized Onions

My grandmother didn’t caramelize the onions for her version of this recipe, but I really love the sweetness and rich flavor they add once they are sauteed and browned. Once they are browned, add in the thyme and nutmeg to bloom the spices in the oil before they are added to the casserole dish.

Cook the onions in a medium skillet with two teaspoons of oil. If you notice that they are browning faster than they are softening you can add in a couple tablespoons of water to help them to soften.

When I make this recipe on the stove in my studio (which is hotter than the stove in my house) I had to add water twice to keep the onions from browning too much. You’ll know they are softened when they take on a translucent appearance.  

Make Breadcrumb Mixture

The breadcrumb mixture is made right in the food processor.

  1. Cut the crusts off first and break the pieces into a few smaller pieces.
  2. Set up the food processor with the steel blade attachement.
  3. Pulse several times to chop the bread into large coarse crumbs. 
  4. Remove the lid and add in the seasonings (paprika and garlic powder) and olive oil.
  5. Process again to moisten the crumbs with the oil and coat them with the spices. 

Assemble the Casserole

  1. Arrange the fish in the casserole dish preferably so that the pieces do not overlap. For a thinner filet like haddock or tilapia it is okay for them to overlap, but for a thicker filet like mahi mahi, halibut or cod, they should be side by side. 
  2. Spread the caramelized onions on top of the fish.
  3. Top the casserole evenly with the breadcrumb mixture.
  4. Pour in the wine
  5. Top with the cheese

Step By Step Photos

To Bake And Serving Tips

Transfer the casserole to the oven and let it bake until the wine is bubbling, the breadcrumbs are golden brown and the cheese is melted. The fish will flake when tested with a fork. 

To serve, use a slotted spatula and leave the wine sauce behind in the casserole dish. Serve over rice or with potatoes. I like to serve asparagus with it as a side dish. It also pairs well with broccoli and brussels sprouts

More Recipes Like This

  • My Baked Fish Sticks recipe is comforting and a total childhood throwback but made from scratch (instead of from a freezer box.)
  • If you have wondered how to perfect pan fried fish, this recipe is a good one to try. It was developed by Molly Stevens and is featured in her latest cookbook. 
  • If you love casseroles, this Chicken and Rice Casserole is a fan favorite. Mexican Chicken casserole is another one to try and a family friendly way to do Mexican night. 
  • If you love Gruyere Cheese then this Butternut Squash Mac and Cheese will be right up your ally! It is like mac and cheese for grown ups! 

fish with toppings and caramelized onion

I developed a slightly different variation of this recipe years ago for EatingWell magazine and their Serves Two cookbook.

Thanks so much for reading. If you make this recipe, let me know by leaving a star rating and review.

Happy Cooking!



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close up of fish casserole

Fish Casserole

  • Author: Katie Webster
  • Total Time: 40 minutes
  • Yield: 6 servings 1x


My grandmother’s recipe for fish casserole with Swiss cheese, caramelized onions, bread crumbs and white wine.


  • 1 tablespoons unsalted butter, softened
  • 1.75 pound white fish fillet, Best Choices in your area
  • ¾ teaspoon salt
  • ½ teaspoon pepper
  • 4 teaspoons extra-virgin olive oil, divided
  • 1 large sweet onion, thinly sliced
  • ½ teaspoon dry thyme, or 1 teaspoon chopped fresh thyme
  • pinch nutmeg
  • ¾ cup dry white wine* See sub note below
  • 2 slices bread, crusts removed preferably sourdough or multi-grain
  • ½ teaspoon paprika
  • ¼ teaspoon garlic powder
  • 1 cup shredded Emmentaler, Gruyere or Swiss cheese


  1. Grease 9 by 13-inch casserole dish with softened butter. Preheat oven to 400 degrees F.
  2. Arrange fish fillet in the prepared casserole dish. Sprinkle with salt and pepper.
  3. Heat 2 teaspoons oil in a medium heavy skillet over medium-high heat. Add onion and cook, stirring often (adding 1 to 2 tablespoons water if necessary to prevent the onion from darkening too much) until the onion softens and starts to brown, 6 to 9 minutes. Stir in thyme and nutmeg. Spread the onion mixture over the fish. Pour wine into the casserole dish.
  4. Pulse bread in food processor until coarse crumbs. Drizzle on the remaining 2 teaspoons oil and add paprika and garlic powder. Process to combine until the bread is evenly red and moist. Sprinkle crumbs over the onion.
  5. Top the casserole with the cheese and place in the oven. Bake until the fish is cooked through and the crumbs are browned, 18 to 22 minutes. Serve warm.


Instead of wine you can use broth (or clam juice) plus one tablespoon lemon juice for a non-alcoholic option.

  • Prep Time: 20 minutes
  • Cook Time: 30 minutes
  • Category: Main Course
  • Method: stove top/Oven
  • Cuisine: American


  • Serving Size: 1/6th casserole
  • Calories: 270 calories
  • Sugar: 2 g
  • Sodium: 443 mg
  • Fat: 11 g
  • Saturated Fat: 5 g
  • Carbohydrates: 10 g
  • Fiber: 1 g
  • Protein: 28 g

Keywords: fish casserole