In a large skillet over medium heat, cook the ground beef until browned. Drain excess fat.
Add onion, bell pepper, and garlic. Cook until vegetables are tender, about 5 minutes.
Stir in ketchup, tomato paste, brown sugar, Worcestershire sauce, mustard, salt, and pepper.
Reduce heat to low and simmer for 10-15 minutes, stirring occasionally.
Serve the mixture on toasted hamburger buns.