Melt the butter. Add flour, salt and 2 tablespoons of hot water to it. Stir and place in a baking dish. Make several punctures on the surface of the dough with a fork and place the pie crust in the oven for 20 minutes.
Soak the mushrooms and boil until tender in salted water for about 20 minutes. Also boil the spinach for 15 minutes in salted water. Then drain them and dry them separately in a frying pan.
Chop and fry the onion. Grate the cheese.
Remove the pastry from the oven, add the spinach and pour over half the cream. Then add the fried onions and mushrooms and sprinkle with nutmeg. Pour in the second half of the cream. Sprinkle the dish with grated cheese and place in the oven for 20 minutes.
Immediately transfer the finished pie from the pan to a plate.