In a large saucepan or Dutch oven over medium-high heat, combine beef and onion.Cook until the meat is mostly browned, about 5 minutes.Drain well.
Meanwhile, in a small bowl, whisk together tomato sauce, ketchup, brown sugar, mustard, vinegar, Worcestershire, 1/4 teaspoon salt, and 1/8 teaspoon pepper.
Pour sauce ingredients over drained beef and stir until evenly coated.Simmer uncovered 20 minutes, stirring occasionally.Cover if you prefer a sloppier Sloppy Joe.Serve in buns.
