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.
In large skillet over medium heat, cook bacon until crisp. Remove bacon from skillet; 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 chicken is fork-tender and juices run clear.
Place toasted bread slices on ungreased cookie sheet; spread each slice with 1 teaspoon steak sauce. Top each with chicken, bacon slices, onion and cheese. Broil 4 to 6 inches from heat for 1 to 2 minutes or until cheese is melted.
673 calories, 45 grams fat, 5 grams carbohydrates, 60 grams proteinper serving. This recipe is low in carbs.