Heat grill. 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. Spread chicken with mustard; sprinkle with pepper.
When ready to grill, place chicken on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium-high coals. Cook 8 to 10 minutes or until chicken is fork-tender and juices run clear, turning once.
Spread cut sides of buns with dressing. Layer lettuce, tomato and chicken on bottom half of buns; cover with top half of buns.