Heat gas or charcoal grill. In 1-quart saucepan, combine barbecue sauce, onion and chiles; mix well.
When grill is heated, lightly brush sauce mixture on one side of each chicken breast half. Place chicken, sauce side down, on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium coals. Lightly brush sauce over chicken. Cook covered 10 to 16 minutes or until juice is clear when center of thickest part is cut (170°F).
Heat remaining sauce to a boil; boil 1 minute. Serve chicken with sauce.