Heat oven to 400 degrees.Mix 2 cups of the cheese,chicken,onion and broccoli in greased pie plate,10 x 1 1/2". Beat remaining ingredients except cheese with wire whisk or hand beater until almost smooth,about 1 minute.Pour into plate. Bake until knife inserted in center comes out clean,25 to 30 minutes Top with remaining cheese.Bake just until melted,1 or 2 minutes.Cool 5 minutes.Serves 6 to 8.