Cook onions in large covered skillet sprayed with cooking spray on medium-high heat 2 minutes; stir, then push to one side of skillet. Add chicken; cook 4 minutes. Turn chicken; top with barbecue sauce. Cook, covered, on medium-low heat 6 minutes or until chicken is done (165°F). Stir onions.
Sprinkle chicken with cheese; cook, covered 1 minute or until cheese is melted.