In a skillet, cook the sausage and onion over medium heat until meat is no longer pink and onion is tender; drain. In a bowl, combine cornmeal, flour, baking powder and salt. Add the corn, milk, eggs and oil. Pour half into a greased 10-in. ovenproof iron skillet. Sprinkle with the sausage mixture and cheese. Spread remaining cornmeal mixture on top. Bake at 425° for 45-50 minutes or until a toothpick inserted in the cornbread comes out clean.