1Between pieces of plastic wrap or waxed paper, place each chicken breast smooth side down; gently pound with flat side of meat mallet or rolling pin until about 1/4 inch thick.
2Spray 8- or 10-inch skillet with cooking spray; heat over medium-high heat. Cook chicken in skillet 10 to 15 minutes, turning once, until chicken is no longer pink in center. Remove chicken from skillet; keep warm.
3In skillet, place rolls, cut sides down. Cook over medium heat about 2 minutes or until toasted. Place chicken on bottom halves of rolls. Top with onion, tomato, dressing, cheese, lettuce and tops of rolls.