Keto Fish Casserole with Mushrooms and French Mustard bakes white fish under a creamy Dijon mushroom sauce and a double layer of melted shredded cheese. A bowl of olive oil cauliflower mash on the side catches every last drop of sauce. Six servings at 5g net carbs each.
Preparation Time: 1 hour Cooking Time: 50 minutes Servings: 6
Ingredients
For the Casserole:
- 25 oz (708g) white fish fillets (such as cod or halibut)
- 15 oz (425g) fresh mushrooms, sliced
- 2 cups (473ml) heavy whipping cream
- 3 oz (85g) unsalted butter (approx. 6 tbsp)
- 2 tbsp (30g / 1.06 oz) Dijon mustard
- 8 oz (225g) shredded cheese (such as Gruyere or cheddar)
- 0.1 oz (3g) fresh parsley, finely chopped (approx. 1 tbsp / 15 ml)
- 1 tsp (5g / 0.18 oz) sea salt (plus more to taste)
- 0.5 tsp (1g / 0.04 oz) black pepper
For the Cauliflower Mash:
- 20 oz (567g) cauliflower florets
- 3 oz (85ml) olive oil (approx. 6 tbsp)
- Salt and pepper to taste
Directions
- Preheat the oven to 350°F (175°C) and lightly grease a 9x13 inch baking dish.
- Melt the 3 oz (85g) of butter in a large skillet over medium-high heat, add the 15 oz (425g) of sliced mushrooms, and fry for 5 minutes until golden brown.
- Stir the 0.1 oz (3g) of chopped parsley, 1 tsp (5g) of salt, and 0.5 tsp (1g / 0.04 oz) of black pepper into the browned mushrooms.
- Reduce the heat to low, pour in the 2 cups (473ml) of heavy whipping cream, add the 2 tbsp (30g / 1.06 oz) of Dijon mustard, and allow the sauce to simmer gently for 10 minutes until slightly thickened.
- Season the 25 oz (708g) of white fish slices lightly with salt and pepper, then arrange them in an even layer at the bottom of your prepared baking dish.
- Sprinkle 6 oz (170g) of the shredded cheese evenly over the raw fish slices.
- Pour the creamy mushroom and mustard sauce completely over the fish and cheese layer.
- Top the casserole evenly with the remaining 2 oz (55g) of shredded cheese and bake in the preheated oven for 30 to 35 minutes until the fish flakes easily and the cheese is bubbly and golden.
- While the casserole is baking, bring a large pot of salted water to a rolling boil, add the 20 oz (567g) of cauliflower florets, and boil for 5 minutes until completely tender.
- Drain the cauliflower florets thoroughly to remove excess moisture and transfer them to a large mixing bowl.
- Pour the 3 oz (85ml) of olive oil into the strained florets, mash thoroughly until smooth and creamy, season with salt and pepper to taste, and mix well before serving alongside the hot fish casserole.
Nutritional Information (Per Serving)
Calculated for 1 serving (1/6th of the recipe)
- Calories: 828 kcal
- Total Carbohydrates: 8g (0.28 oz) (Net Carbs: 5g / 0.18 oz)
- Protein: 39g (1.38 oz)
- Fat: 71g (2.5 oz)
Chef's Pro Tips for the Best Keto Fish Casserole
To make sure this dish comes out perfectly every single time, here are a few tricks from my kitchen:
- Pat the Fish Dry: White fish holds a lot of water. To prevent your creamy Dijon sauce from splitting or becoming too thin, thoroughly pat your fish fillets dry with paper towels before seasoning them.
- Brown the Mushrooms Beautifully: Don't rush the mushrooms! Let them fry undisturbed for a few minutes to develop a deep, golden-brown crust. This adds an incredibly savory, umami flavor to the final sauce that you just can't get from steaming them.
- Squeeze the Cauliflower: Cauliflower absorbs water like a sponge when boiled. For the creamiest, least watery mash, drain the florets well and gently squeeze out excess moisture using a clean kitchen towel before blending them with the olive oil.
- Broil for a Golden Crust: If your casserole is cooked through but the cheese on top isn't quite as browned as you'd like, switch your oven to the broiler setting for the final 2-3 minutes. Keep a close eye on it so it doesn't burn!
Frequently Asked Questions (FAQ)
Can I use frozen fish for this keto fish casserole? Yes, you can absolutely use frozen white fish! However, it is crucial that you thaw the fish completely in the refrigerator overnight and pat it exceptionally dry with paper towels before assembling the casserole. If you use partially frozen fish, it will release too much water during baking and dilute the creamy mushroom sauce.
What is the best white fish for baking in a casserole? For baked casseroles, firm white fish varieties are the best choice because they hold their shape well under heavy sauces and don't turn to mush. Cod, halibut, haddock, and pollock are excellent options for this recipe. Avoid very delicate fish like flounder or sole.
How do I store and reheat this low carb fish bake? Store any leftover keto fish casserole in an airtight container in the refrigerator for up to 3 days. To reheat, cover the dish with foil and warm it in the oven at 300°F (150°C) until heated through. Microwaving is an option, but do it gently on medium power to prevent the cream sauce from separating and the fish from becoming rubbery.
Is the cauliflower mash in this recipe dairy-free? Yes! While traditional mashed potatoes or keto cauliflower mashes rely heavily on butter and cream cheese, this specific side dish uses olive oil to achieve a smooth texture, making the mash portion entirely dairy-free.
Pro Tips
- Let the sliced mushrooms fry undisturbed for a few minutes so they develop a deep golden-brown crust. That crust is where the umami depth of the sauce comes from.
- Pat the white fish fillets dry before seasoning them. White fish holds a lot of water, and releasing it into the sauce will make it split or go thin.
- Squeeze the boiled cauliflower in a clean kitchen towel before mashing it with the olive oil. Cauliflower soaks up water like a sponge, and this is the path to a creamy, not watery, mash.
- Sprinkle the first 6 oz of cheese directly over the raw fish before pouring on the sauce, so it melts into a savory layer beneath the mushrooms.
- If the top isn't browned enough once the fish is cooked through, switch the oven to broil for the final 2-3 minutes and watch it closely.
Frequently Asked Questions
Which cheese works best in this casserole?
The recipe calls for shredded Gruyere or cheddar. Both melt smoothly under the creamy mustard sauce; shred from a block for the best melt.
Can I use frozen cauliflower for the mash?
Yes. Boil it until tender, then squeeze out the excess moisture especially thoroughly before mashing it with the olive oil.
Can I prepare the casserole ahead of time?
Brown the mushrooms and simmer the cream-mustard sauce earlier in the day, then assemble the dish and bake it when you're ready for dinner.
How many net carbs are in a serving?
Each of the six servings has 8g total carbs and 5g net carbs, alongside 39g of protein from the fish and cheese.
Recipe card below.
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Ingredients
Method
- Preheat the oven to 350°F (175°C) and lightly grease a 9x13 inch baking dish.
- Melt the 3 oz (85g) of butter in a large skillet over medium-high heat, add the 15 oz (425g) of sliced mushrooms, and fry for 5 minutes until golden brown.
- Stir the 0. 1 oz (3g) of chopped parsley, 1 tsp (5g) of salt, and 0. 5 tsp (1g / 0.04 oz) of black pepper into the browned mushrooms.
- Reduce the heat to low, pour in the 2 cups (473ml) of heavy whipping cream, add the 2 tbsp (30g / 1.06 oz) of Dijon mustard, and allow the sauce to simmer gently for 10 minutes until slightly thickened.
- Season the 25 oz (708g) of white fish slices lightly with salt and pepper, then arrange them in an even layer at the bottom of your prepared baking dish.
- Sprinkle 6 oz (170g) of the shredded cheese evenly over the raw fish slices.
- Pour the creamy mushroom and mustard sauce completely over the fish and cheese layer.
- Top the casserole evenly with the remaining 2 oz (55g) of shredded cheese and bake in the preheated oven for 30 to 35 minutes until the fish flakes easily and the cheese is bubbly and golden.
- While the casserole is baking, bring a large pot of salted water to a rolling boil, add the 20 oz (567g) of cauliflower florets, and boil for 5 minutes until completely tender.
- Drain the cauliflower florets thoroughly to remove excess moisture and transfer them to a large mixing bowl.
- Pour the 3 oz (85ml) of olive oil into the strained florets, mash thoroughly until smooth and creamy, season with salt and pepper to taste, and mix well before serving alongside the hot fish casserole.
- Pat the Fish Dry: White fish holds a lot of water. To prevent your creamy Dijon sauce from splitting or becoming too thin, thoroughly pat your fish fillets dry with paper towels before seasoning them.
- Squeeze the Cauliflower: Cauliflower absorbs water like a sponge when boiled. For the creamiest, least watery mash, drain the florets well and gently squeeze out excess moisture using a clean kitchen towel before blending them with the olive oil.
