Cut the mushrooms into plates, boil for 10 minutes and discard in a colander. Cut the onion in half rings, fry in vegetable oil until soft, then add the mushrooms and fry for about 10 minutes.
Cut tomatoes and fry separately. Sliced cheese on a fine grater.
Put mushrooms with onions in a baking dish, put tomatoes on top and sprinkle with cheese. Bake in the oven at 180 degrees for 10 minutes.