Cook potatoes and peel. Smash on puree with the butter, salt and pepper. Boil chicken breasts and tear or cut into pieces. In a round pan greased with butter, arrange in the middle row of the meat then by mashed potatoes and alternate so to finish the products. Fried mushrooms and place on top, then sprinkle with grated cheese. Bake in moderate oven until the cheese is melted and browned.