This old-school Coney sauce is perfect for a real New York-style dog.
Cook the beef and onion in a big skillet over medium-high. Stir it around so the beef crumbles, until it's browned. Drain the beef, and transfer it and the onion to a slow cooker. Stir ketchup, sugar, vinegar and mustard into the slow cooker pot, then season with celery seed, Worcestershire sauce, pepper and salt. Put the cover on let it simmer on Low for at least two hours before serving.