In a skillet, cook sausage and onion over medium heat until meat is no longer pink; drain. Place in a lightly greased 13x9-in. baking dish. Layer with the rice, cereal and cheddar cheese. In a bowl, beat the eggs, soup and milk. Spread over top. , Bake, uncovered, at 350° for 55-60 minutes or until a knife inserted in the center comes out clean. Let stand for 5 minutes before cutting. Refrigerate any leftovers.