Cook the beef and onion in an 8-quart sauce pot over medium high heat until the beef is well browned, stirring often to break up the meat. Pour off any fat.
Stir the sauce, ziti and 2 cups mozzarella cheese in the sauce pot. Spoon the beef mixture into 2 (12 1/2 x 8 1/2 x 2-inch) disposable foil pans. Top with the remaining mozzarella and Parmesan cheeses.
Bake at 350°F. for 30 minutes or until the beef mixture is hot and the cheese is melted.
Tip: To make ahead and freeze, prepare the ziti as directed above but do not bake. Cover the pans with foil and freeze. Bake the frozen ziti, uncovered, at 350°F. for 1 hour or until it's hot. Or, thaw the ziti in the refrigerator for 24 hours, then bake, uncovered, at 350°F. for 45 minutes or until it's hot.
Serving Suggestion: Serve with zucchini cut into thinly sliced ribbons (use vegetable peeler) and sautéed in olive oil. Sprinkle with grated parmesan. For dessert serve lemon sorbet with a mint sprig.