Cook pasta in large Dutch oven or pot according to package directions. Drain and return to Dutch oven.
Meanwhile, saute chopped onion in hot oil in a large skillet over medium high heat 5 minutes or until tender. Add garlic, and saute one minute. Add beef, and cook, stirring until beef crumbles and is no longer pink. Drain beef mixture, and return to pan. Stir in pasta sauce and 1/2 tsp salt. Set aside.
Melt butter in a heavy saucepan over low heat; whisk in flour until smooth. Cook, whisking constantly, one minute. Gradually whisk in milk; cook over medium heat, whisking constantly, until mixture is thickened and bubbly. Stir in Parmesan cheese, remaining 1/4 tsp. salt, and pepper. Pour sauce over pasta in Dutch oven, stirring until pasta is evenly coated.
Transfer pasta mixture to a lightly greased 13X9 baking dish. Top evenly with beef mixture; sprinkle evenly with mozzarella cheese.
Bake at 350 degrees for 20-25 min. or until cheese is melted. Let stand 10 min. before serving.