Butter a metal oven-dish with wavy edges of about 8 inches in diameter; line with the sheet of dough to a thickness of about 0,2 inch, prick all over, cover with some thin white paper, fill with dried peas, and put into a moderate oven. Remove after 15-20 minutes (it should be barely colored), remove the peas and the sheet of paper, leaving the crust in the oven-dish. Brush with beaten egg and leave in the oven doorway for a couple of minutes to dry the pastry. After this procedure, the crust may be filled. Cut the mushrooms into thick slices, sauté them in the butter for some seconds, add the chopped mixture of onion and garlic, mix, and as soon as the mixture is lightly golden, douse with Marsala wine. Allow to evaporate and reduce the mixture. At this point, pour in the cream, season with salt and pepper, continue cooking over high heat for 5 minutes, taking care to mix often, and then mix in the light béchamel. Cook for some seconds, then fill the crust with this mixture, and sprinkle with grated Parmesan and melted butter. Put into a hot oven. Serve when the surface has become nicely golden.