Preheat the oven to 400 degrees F (200 degrees C). Grease of shortening the floor and walls of a baking dish rectangular 13 x 9 x 2 inches (33 x 23 x 5 cm). Sprinkle the bottom of the dish of broccoli, 2 cups of cheese, chicken and onion. Mix all the remaining ingredients until homogeneity. Pour on the preparation to the chicken. Bake uncovered 40 to 45 minutes or until a knife inserted in the center comes out clean. Sprinkle with the remaining cup of cheese. Bake for 1 to 2 minutes or until the cheese has melted. Let stand 5 minutes before cutting.