Place 1 chicken breast half between 2 pieces of plastic wrap or waxed paper. Working from center, gently pound chicken with flat side of meat mallet or rolling pin until about 1/4 inch thick; remove wrap. Repeat with remaining chicken breast halves.
Cook bacon in large skillet over medium heat until crisp. Remove bacon; drain on paper towels. Reserve 1 tablespoon drippings in skillet. Add onion to drippings; cook and stir 2 to 4 minutes or until tender. Remove onion from skillet. Add chicken to skillet; cook 6 to 8 minutes on each side or until lightly browned and juices run clear.
Place toasted bread slices on cookie sheet; spread each slice with 1 teaspoon steak sauce. Top with chicken, bacon slices, onion and cheese. Broil 4 to 6 inches from heat for 1 to 2 minutes or until cheese is melted.