1. Cook ground round and onion in a large skillet, stirring until beef crumbles and is no longer pink; drain meat, and return to skillet.
2. Spoon half of meat mixture into a lightly greased 5-qt. slow cooker. Top with spaghetti. Spoon remaining half of meat mixture over top.
3. Combine pasta sauce, diced tomatoes, 1/2 cup water, and Italian seasoning. Pour mixture over meat, and sprinkle with Cheddar cheese.
4. Cover and cook on Low 4 hours. Let stand 10 minutes before serving. Serve with Parmesan cheese.
Note: We tested with McCormick Perfect Pinch Italian Seasoning.