Finely chopped onion stew in half butter, on it is dusted flour and fry. Make slow 1/4 liters of water and stir the sauce until it is smooth. Add wine and grated chocolate, seasoned with cinnamon, cloves, salt and pepper. Suitable fireproof container is coated well with butter and place on an asbestos pad on low heat. Cut into portions fish fillet is put inside, pour the sauce and stew slowly until it is ready. Clean, sliced ​​mushrooms smothered in butter and rest 10 minutes before serving pour over the fish. Serve with steamed rice.