Boil the imperial vegetables in water until they are al dente. Bring the peas to the boil in water so that they are no longer frozen. The potatoes cook with a good bite, peel and quarter very large ones, halve the smaller ones. Do not cook the vegetables completely, as they will cook to the end in the oven and otherwise disintegrate.
Clean and quarter the mushrooms and fry them in a little butter. Add 2 cups of cream, season heartily and reduce a little.
Spread a few flakes of butter in a baking dish so that nothing cakes. Place the potatoes in it, place the vegetables and peas on top and distribute the mushroom sauce evenly on top.
Sprinkle the grated cheese on top and bake the casserole in the oven at 180 degrees for 20-30 minutes until the cheese melts well and is golden.