Carefully separate the fillet with the skin from the bone and beat off.
Fry mushrooms with onions in vegetable oil.
Put the chicken fillet on a baking sheet drizzled with oil. Put the mushrooms on half of the fillet and cover with the second half of the fillet, lightly fasten with toothpicks.
Bake for about 20 minutes at 180C, and then sprinkle with grated cheese and bake a little more. When serving, garnish with chopped tomato wedges and herbs.