Preheat oven to 375. Prepare a 8" square dish with cooking spray; set aside. In a mixing bowl, combine chicken, half cup onions, crushed crackers, egg, water, chili powder, and salt. Mix gently but thoroughly. Press into prepared dish. Bake for 30 minutes, or until chicken is no longer pink. Meanwhile, to prepare potato topping, combine mashed potatoes, corn kernels, and remaining onions in a mixing bowl. Mix well. Spread over chicken mixture in dish. Sprinkle with cheese. Bake five minutes more, or until heated through.