In a large bowl, combine broccoli, 2 cups cheese and onion. In another bowl, combine eggs, milk, biscuit mix, salt and pepper; mix well. Pour over broccoli mixture; toss gently. Pour into two greased 9-inch pie plates. Bake at 400° for 25-30 minutes or until a knife inserted in center comes out clean. Sprinkle with remaining cheese; return to the oven for 1-2 minutes or until melted. Let stand 5-10 minutes before cutting.