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, vinegar, Worcestershire, mustard, and salt and pepper to taste (I like ¼ teaspoon salt and ⅛ 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.
