In a large skillet, cook beef and onion over medium heat until the meat is no longer pink; drain. Add spaghetti sauce. Cover and simmer until heated through. Pour into a greased 13x9-in. baking dish. Sprinkle with cheese.
In a blender, combine the milk, eggs, oil, flour and salt; cover and process until smooth. Pour over cheese. Bake, uncovered, at 400° for 25-30 minutes or until golden brown.