1. Place onion in a 5-qt. slow cooker. Crumble ground beef over onion, and add tomatoes. Cover and cook on High for 4 hours or on Low 8 hours. Break up any large pieces of beef with a wooden spoon; stir in salt. Remove half of sauce, and reserve for another use; keep remaining sauce warm in slow cooker.
2. Cook penne pasta according to package directions. Drain and transfer to 4 shallow bowls. Spoon meat sauce over each portion of pasta. Serve with Parmesan cheese, chopped fresh basil, and freshly ground black pepper to taste.