Keto fish casserole with mushrooms and French mustard

So hearty. So satisfying. And so, so simple to make. If you love fish, mushrooms and creamy, rich sauces, be prepared to fall head over heels in love with this amazing keto casserole.
So hearty. So satisfying. And so, so simple to make. If you love fish, mushrooms and creamy, rich sauces, be prepared to fall head over heels in love with this amazing keto casserole.
6 servingservings


  • 1 lb 450 g mushrooms
  • 3 oz. 90 g butter
  • 1 teaspoon 1 teaspoon salt
  • pepper, to taste
  • 2 tablespoons 2 tablespoons fresh parsley
  • 2 cups 475 ml heavy whipping cream
  • 2 tablespoons 2 tablespoons Dijon mustard
  • 8 oz. 225 g shredded cheese
  • 1½ lbs 700 g white fish, for example cod
  • 113 lbs 600 g broccoli or cauliflower
  • 3 oz. 90 g butter or olive oil


Instructions are for 6 servings. Please modify as needed.

  1. Preheat the oven to 350°F (175°C).
  2. Cut the mushrooms into wedges. Fry in butter until the mushrooms have softened, about 5 minutes. Add salt, pepper, and parsley.
  3. Pour in the heavy cream and mustard and lower the heat. Let simmer for 5-10 minutes to reduce the sauce a bit.
  4. Season the fish with salt and pepper and place in a greased baking dish. Sprinkle 3/4 of the cheese on and pour the creamed mushrooms on top. Top with the remaining cheese.
  5. Bake for about 30 minutes if the fish is frozen, or slightly less if it’s fresh. Probe with a sharp knife after 20 minutes; the fish is done if it flakes easily with a fork. And remember that the fish will continue to cook even after you have taken it out of the oven.
  6. Meanwhile, make the side dish. Cut the broccoli or cauliflower into florets. Boil in lightly salted water for a few minutes. Strain off the water and add olive oil or butter.
  7. Mash coarsely with a wooden spoon or fork.
  8. Season with salt and pepper and serve with the fish.


Did you know that the stem of broccoli and cauliflower is wonderful, too? Simply peel the rough outer layer off with a sharp knife or a potato peeler. And then cut the stems lengthwise into rods, or dice them. Voila! More cruciferous love!


  1. Nichy
    This was delicious. I'm not a seafood fan but thought I'd make this, mainly for my hubby, and I really enjoyed it.
  2. Jenny
    I thought this recipe would be a bit bland with plain white fish, so used smoked cod to add a bit more flavour. It turned out so well.
  3. Brenda
    Absolutely gorgeous, made it with freshly landed north sea cod. I used fennel and basil to flavour the sauce.
  4. Vicki
    Wondering what to use instead of mushrooms - can't stand them cooked - any suggestions?
  5. Nani
    Vicky, same here.... I'm thinking about doing it tomorrow with leek instead.... what do you think?
  6. Anneline Grobler
    This was a wonderful dish. I don't like fish as a rule but thought it might be a good dish for dinner guests and I really liked it:)
    I did however thicken it up a tad with some Xanthan gum...does anyone know the good or bad about this product on a low carb plan??
  7. Rhonda
    What kind of cheese is everyone using?
  8. Stephen
    Mature cheddar all the way, as its so flavoursome. If you prefer you could use a milder version.
  9. andrea
    omg, was that delicious! used salmon, as its what we had, fattier, and hubby doesn't like whitefish anyway. :D
    Needs more cream, but oh, so delicious!!! definitely a keeper!
  10. Letitia
    Tasted really good, I had my doubts combining mushroom, mustard and fish, but my pangasius turned out fantastic. I used grated gouda 48+. The cream sauce was a bit too thin, maybe the xanthan gum could solve it. So good I would serve it guests too (it looks very delicious).
  11. Paul
    I cooked this for the first time tonight. Used a Fish Pie Pack from the supermarket which had a mix of cod, salmon and smoked haddock. Also threw in a can of tuna. Tasted amazing, but as others have said the sauce could have been thicker. Will definitely make again!

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