PREHEAT oven to 400°F. Line a 13x9x2-inch baking pan with Reynolds Wrap® Heavy Duty Aluminum Foil.
PLACE onion slices evenly in foil-lined pan. Top onions with chicken, peppers and pasta sauce.
BAKE 20 minutes. Remove from oven sprinkle with cheese and pepperoni.
CONTINUE BAKING 20 to 25 minutes longer or until chicken reaches 170°F and juices run clear.