Put broccoli (clean fresh and cut into small florets, frozen not defrosted) in a baking dish.
Put the mushrooms (clean fresh, cut into slices; frozen, not defrosted, sliced) in the baking dish. Peel the potatoes, cut into small cubes and add to them.
Mix the crème fraîche with the cream, cornstarch, salt, pepper and about 1/3 of the cheese. Then add to the potato-vegetable mixture and mix everything together.
Sprinkle with the rest of the cheese and bake for about 80 - 90 minutes uncovered at 160 ° C convection until golden yellow. With top / bottom heating 200 ° C.