HEAT grill to medium-high heat.
GRILL chicken and onions 8 min., turning onions frequently. Transfer onions to bowl. Turn chicken; grill 7 to 9 min. or until done (165F), turning occasionally.
CUT chicken into strips. Add barbecue sauce to onions; toss to coat.
FILL bread slices with chicken, cheese and onions.
Serving Suggestion: Serve with a fruit salad and assorted cut-up fresh vegetables.
Substitute: Prepare using whole wheat hamburger buns.