Fry the onion with mushrooms and garlic in vegetable oil until soft. Add the vegetable mixture and cook over medium heat until done.
In a saucepan, melt the cream cheese with grated mozzarella, pour in the milk, add nutmeg, salt and pepper to taste.
Add breadcrumbs to the vegetable mixture and add cheese sauce. If the mixture is too runny, add more breadcrumbs.
Roll out the dough into 2 layers. Using a plate, cut out a large circle and cut out round holes in the center. Spread the filling over one layer and cover with the other. Fix the edges. Brush the dough with beaten egg. Bake at 190 °C for 20-25 minutes.