In a large skillet over medium heat, cook the ground beef, onion, and bell pepper (if using) until the meat is browned and the vegetables are soft. Drain any excess fat.
Stir in the garlic and cook for another minute until fragrant.
Add the ketchup, mustard, brown sugar, and Worcestershire sauce to the skillet. Simmer the mixture for about 5-10 minutes until thickened. Season with salt and pepper to taste.