Keto fish casserole with mushrooms and French mustard

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.

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.
6 servingservings


  • 1 lb 450 g mushrooms
  • 3½ oz. 100 g butter
  • 1 tsp 1 tsp salt
  • pepper, to taste
  • 2 tbsp 2 tbsp fresh parsley
  • 2 cups 475 ml heavy whipping cream
  • 2 tbsp 2 tbsp Dijon mustard
  • 8 oz. 225 g (475 ml) shredded cheese
  • 1½ lbs 650 g white fish, for example cod
  • 1½ lbs 650 g broccoli or cauliflower
  • 3½ oz. 100 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!

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  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?
    Reply: #14
  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!
  12. Jon
    It smells and taste very good... but i didnt get casserole.. Its real soupy, and I simmered the heavy cream for a long time to. Not sure what i did wrong.
  13. Jim
    Made it tonight and followed instructions to the letter!
    Amazing 😉
  14. Una

    What kind of cheese is everyone using?

    Rhonda, I am.making this this week and will use the strongest mature cheddar we have in England - No 7 on the packet - either Canadian or English. It's grated in Britain not American shredded!!! 😂 Needs to be a hard cheese not anything that is remotely soft. I never buy already grated cheese as the strength only goes up to No 4 so too mild and easy to grate cheese. Another cheese I would think of using is a good quality Gruyere, which is also a hard and strong delicious cheese. Hope all this helps. Thanks to.everyone who said the sauce was too thin. Will bare that in mind when cooking this.

  15. Mauretta
    This was amazing, My hubby fishes alot so we always have fish in the house and always looking at different ways to cook it and this was so nice! i served mine with cauliflower mash divine!!
  16. Aidan
    Delicious! Favourite dish.
  17. Sheri Abear
    Delicious 😋♥️I used Colby/Jack cheese..
  18. Jarrah
    As Jon said, it was very soupy. Loved the flavour but it wasn’t a casserole
  19. Jennifer
    This was so good. Modified and added some cauli rice since I didnt have enough mushrooms. Sauce thickened beautifully. I also hit it with about 5 minutes of broiler for a crispy top. Two thumbs up from the husband too!
  20. Hollly
    I used an oven friendly saucepan and cooked it on the stove top until the sauce was thickened. Added some cheese to the top and under the broiler for a couple minute. Used a mix of several cheeses but mostly cheddar. Got a thumbs up!
  21. Anneleen
  22. Monika
    simply fantastic :-)
  23. Monica
    Could I substitute coconut cream?
    Reply: #24
  24. Kristin Parker Team Diet Doctor

    Could I substitute coconut cream?

    You can try that, it may give a coconut flavor to the dish you may not enjoy but coconut cream is a common sub for heavy cream.

  25. Carlye Brownell
    How many cups is one serving?
    Reply: #26
  26. Kristin Parker Team Diet Doctor

    How many cups is one serving?

    That depends on how much your recipe reduces (or doesn't). Final volume can vary. This recipe as written makes 6 servings.

  27. Janet
    we just loved this recipe, thank you.
  28. Rock
    One of our favorite fish recipes! Tastes delicious and easy! Of all things, I add a bay leaf based upon a fish pie recipe in an Irish cookbook. It adds a lot of flavor and complements the mushrooms very nicely.
  29. Ray Bussey
    I added some paprika to the sauce. It was fantastic!
  30. Tara
    Omg this was soooooooo good! Thank you!
  31. Eva
    Very delicate❤️❤️
  32. Sally
    Loved this recipe, used cod as suggested but could use a stronger tasting fish. Deliciousness . 😍❤️

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