Clean the mushrooms and cut them in half. Peel and finely chop the onion. Wash parsley, dab dry, put some aside for garnishing, chop the rest finely. Heat the oil in a coated pan and fry the mushrooms.
Add the onion and fry for about 1 minute. Season to taste with salt and pepper and add parsley. Wash the tomato, grate dry and cut into slices. Cover the slice of bread with the tomatoes and spread mushroom vegetables on top.
Garnish with set aside parsley.