PREHEAT oven to 350° F. Place Crispy Fried Onions into plastic bag; crush lightly with a rolling pin. Transfer to a sheet of waxed paper.
MIX mayonnaise, mustard and Parmesan cheese in pie plate. Dip chicken into mayonnaise mixture, then into crumbs, pressing gently to coat well.
PLACE chicken into a foil-lined 10-inch x 15-inch baking dish. Bake 25 minutes or until chicken is no longer pink in center. Garnish with parsley, if desired.