Flatten chicken breasts to ?½ inch. Heat butter in a skillet and sauté breasts over low heat until golden brown and tender. Remove chicken and keep warm. Add mushrooms to pan and sauté until tender. Pour in sherry and simmer another minute. Place chicken in an broiler proof dish/skillet and top with mushrooms/sherry. Place 1-2 slices mozzarella cheese on chicken pieces and broil til lightly melted. Delicious served with wild rice.