Heat oven to 375° F. Cook pasta according to package directions; drain.
Combine ground beef, 2 tablespoons breadcrumbs, onion and salt in medium bowl. Form into 8 (1 ½-inch) meatballs.
Heat oil in 10-inch nonstick skillet. Brown meatballs on all sides; drain grease. Reduce heat to low. Add spaghetti sauce. Cover; cook 12-14 minutes or until meatballs are cooked through. Stir in cooked pasta.
Spray 1 ½-quart casserole with nonstick cooking spray. Spoon ½ of meatball mixture into casserole. Top with ½ cup shredded Cheddar cheese. Top with remaining meatball mixture and remaining cheese.
Stir together remaining 2 tablespoons breadcrumbs and melted butter. Sprinkle over casserole.
Bake 15-20 minutes or until casserole is heated through.