COOK onions in large covered skillet sprayed with cooking spray on medium-high heat 2 min.; stir, then push to one side of skillet. Add chicken; cook 4 min. Turn chicken; top with barbecue sauce. Cook, covered, on medium-low 6 min. or until chicken is done (165ºF). Stir onions.
SPRINKLE chicken with cheese; cook, covered, 1 min. or until cheese is melted.
FILL rolls with chicken and onions.