1Heat oven to 400°. Grease bottom and sides of rectangular baking dish, 13x9x2 inches, with shortening. Sprinkle broccoli, 2 cups of the cheese, the chicken and onion in baking dish.
2Stir remaining ingredients until blended. Pour over chicken mixture.
3Bake uncovered 40 to 45 minutes or until knife inserted in center comes out clean. Sprinkle with remaining 1 cup cheese. Bake 1 to 2 minutes or until cheese is melted. Let stand 5 minutes before cutting.