If it`s fresh mushrooms, clean them first, otherwise it`s best to buy slices right away. Drizzle fresh mushrooms with lemon juice. Peel and chop the onion. Allow the onions to become translucent in a pan with oil. Add the mushrooms and fry them until all the liquid is gone. Season with marjoram, pepper and salt.
Preheat the oven to 220 ° C.
Toast the toast. Spread the mushrooms on top, tomatoes on top and season with salt and pepper, and finally add cheese (best grated).
Place the topped slices of toast on a baking sheet and bake until the cheese melts.