Grandpa's Classic Coney Sauce

Place the ground beef and onion in a large skillet over medium-high heat. Cook, stirring to crumble, until beef is browned. Drain. Transfer the beef and onion to a slow cooker and stir in the ketchup, sugar, vinegar and mustard. Season with celery seed, Worcestershire sauce, pepper and salt. Cover and simmer on Low setting for a few hours before serving.