Clean the mushrooms, wash if necessary and dab dry. Peel onion and garlic. Chop the onion finely, press the garlic through a garlic press. Heat 20 grams of fat in a large pan and fry half of the mushrooms until golden brown.
Season with salt and pepper and take out. Fry the remaining mushrooms in the remaining fat. Add the onions and garlic shortly before the end of the frying time and fry. Season with salt and pepper and place in an ovenproof dish with the remaining mushrooms.
Wash, clean and slice the tomatoes. Drain mozzarella and cut into slices. Wash basil, dab dry and cut into strips, except for a little to garnish. Cover mushrooms with tomatoes and mozzarella and sprinkle with salt, pepper and basil, except for a tablespoon.
Bake in the preheated oven (electric: 225 °C / gas: level 4) for 20-25 minutes. Sprinkle with remaining basil and serve garnished. Baguette tastes good with it.