In a skillet, cook beef and onion over medium heat until meat is no longer pink; drain. Stir in spaghetti sauce. Cover and simmer for 10 minutes. Stir in spaghetti; set aside.
In a saucepan, melt butter. Stir in flour and salt until smooth. Gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat. Stir in 3/4 cup cheddar cheese and Parmesan cheese until melted.
Place half of the spaghetti mixture in a greased 13-in. x 9-in. baking dish. Top with cheese sauce and the remaining spaghetti mixture. Sprinkle with remaining cheddar cheese.
Bake, uncovered, at 350° for 30 minutes or until heated through. Yield: 10 servings.