Brown the beef in a nonstick skillet over medium heat, 5 to 7 minutes. Add onions and season with salt and pepper. Cook until onions soften, about 7 minutes. Add ketchup, vinegar, Worcestershire sauce, sugar and mustard. Stir to combine. Simmer until slightly thickened, about 10 minutes. Spoon over split toasted hamburger buns.
Per Serving: Serving size: 1 sandwich, 430 calories (170 from fat), 19g total fat, 7g saturated fat, 75mg cholesterol, 890mg sodium, 39g carbohydrates, (2 g dietary fiber, 17g sugar), 26g protein.
Note: We've provided special diet and nutritional information for educational purposes. But remember — we're cooks, not doctors! You should follow the advice of your health-care provider. And since product formulations change, check product labels for the most recent ingredient information. See our Terms of Service.