Sauté chopped onion in hot oil in a large skillet over medium-high heat 5 minutes or until tender. Add garlic, and sauté 1 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 teaspoon salt. Set aside.
Cook pasta in a large Dutch oven according to package directions. Drain and return to Dutch oven.
Melt butter in a heavy saucepan over low heat; whisk in flour until smooth. Cook, whisking constantly, 1 minute. Gradually whisk in milk; cook over medium heat, whisking constantly, until mixture is thickened and bubbly. Stir in Parmesan cheese, remaining 1/4 teaspoon salt, and pepper. Pour sauce over pasta in Dutch oven, stirring until pasta is evenly coated.
Transfer pasta mixture to a lightly greased 13- x 9-inch baking dish. Top evenly with beef mixture; sprinkle evenly with mozzarella cheese.
Bake at 350° for 20 to 25 minutes or until cheese is melted. Let stand 10 minutes before serving.