Put onion in pot of slow cooker. Crumble ground beef on top and pour tomatoes over beef. Cover and cook for 4 hours on high or 8 hours on low. Break up any large pieces of beef with a wooden spoon. Season with salt. Remove half of sauce and reserve for another recipe; keep remaining sauce warm in slow cooker. (Makes about 2 1/2 quarts meat sauce.)
Bring a pot of salted water to a boil. Cook pasta according to package directions, stirring often, until al dente. Drain and transfer to four shallow bowls. Spoon meat sauce over each portion of pasta. Serve with Parmesan cheese, basil and freshly ground pepper.