Chop the onion and bell pepper; mince the garlic.
Brown the ground beef in a large skillet over medium heat, breaking it up as it cooks. Drain excess fat.
Transfer the beef to the slow cooker. Add onion, bell pepper, and garlic. Stir to combine.
Whisk together tomato sauce, ketchup, tomato paste, brown sugar, Worcestershire, mustard, vinegar, chili powder, paprika, salt, pepper, and red pepper flakes.
Pour the sauce into the slow cooker and mix well to coat the beef and vegetables.
Cover and cook on LOW for about 5 hours, stirring once halfway through.
If you prefer a thicker mix, uncover for the last 20 minutes to let it reduce slightly.
Spoon onto toasted hamburger buns and serve warm.