Heat a large skillet over medium-high heat. Cook and stir beef and onion in the hot skillet until beef is completely browned, 5 to 7 minutes. Drain and discard grease. Return skillet to the heat.

Stir ketchup, vinegar, brown sugar, Worcestershire sauce, and mustard into the beef mixture; simmer, stirring occasionally, until the liquid is thick, 15 to 20 minutes.