In a large bowl, combine 2 cups cheese, broccoli, chicken and onion; spread into a greased 10-in. pie plate. In a small bowl, beat the milk, eggs, biscuit mix, salt and pepper until smooth.
Pour over broccoli mixture (do not stir).
Bake at 400° for 30-35 minutes or until a knife inserted near the center comes out clean.
Sprinkle with the remaining cheese.
Let stand for 5 minutes or until cheese is melted.