Preheat oven to 400. Prepare a 2-quart casserole dish with cook spray; set aside. In a large saucepan, cook chicken and onions until chicken is no longer pink. Stir in sloppy joe sauce and cheese. Place mixture into prepared dish. In a mixing bowl, combine baking mix, milk, and egg. Mix well. Pour mixture over chicken mixture in dish. Bake 30 minutes, or until golden brown.