In a skillet, cook garlic in hot butter or margarine for 30 seconds.
In a shallow bowl, stir together the all-purpose flour, the salt, and the pepper. Coat the chicken with the flour mixture, coating evenly on both sides.
Cook the coated chicken breasts in the melted butter in the skillet for 20 minutes or until chicken is tender and no longer pink, turning once. Remove from pan to unheated rack of a broiler pan.
Cook mushrooms and onion in skillet until just tender. Spoon over each chicken breast half. Top with cheese.
Broil the chicken 4 to 5 inches from heat for 1 to 2 minutes or until the cheese melts. Makes 4 servings.