Cook the beef and onion in a 4-quart saucepan over medium-high heat until the beef is well browned, stirring often to break up the meat. Pour off any fat.
Stir the sauce, 1 cup mozzarella cheese and pasta in the saucepan. Spoon the mixture into a 3-quart shallow baking dish. Sprinkle with the remaining mozzarella cheese and Parmesan cheese. Bake at 350°F for 30 minutes or until hot and bubbling.