Heat oven to 400°. Grease 10×1 1/2-inch pie plate. Rinse broccoli under running cold water to thaw; drain thoroughly. Mix broccoli, 2 cups cheese, chicken and onion in plate. Beat milk, eggs, baking mix, salt and pepper until smooth, 15 seconds in blender on high or 1 minute with hand beater. Pour into plate. Bake until knife inserted in center comes out clean, 25 to 35 minutes. Top with remaining cheese. Bake just until cheese is melted, 1 to 2 minutes longer. Cool 5 minutes.