Preheat the oven to 175 degrees Celsius.
Fry the mushrooms in the butter for 5 minutes. Add the salt, pepper and parsley and fry briefly.
Add the cream and mustard and place the pan on a lower heat. Let it simmer for 5 to 10 minutes so that the sauce can thicken.
Season the fish with salt and pepper and place them in a greased baking dish. Sprinkle 3/4 of the cheese on top and put the mushrooms on top. Garnish with the remaining cheese.
If the fish is still frozen, bake the oven dish in the oven for about 30 minutes. If the fish has already thawed, the baking time is shorter. Check the fish after 20 minutes. If you easily pierce the fish with a fork, it is cooked.
In the meantime, prepare the side dish. Boil the broccolli or cauliflower in water with some salt for 5 to 10 minutes. Drain and mix with the tablespoon of butter / olive oil.
Mash the vegetables and season with salt and pepper. Place on a plate with the fish.