In a large skillet over medium heat, brown the ground beef until fully cooked, breaking it apart as it cooks. Drain any excess fat.

Add the diced onion, green bell pepper, and minced garlic to the skillet and sauté for about 5 minutes, until the vegetables are softened.

Transfer the beef and vegetable mixture to a slow cooker.

Stir in the ketchup, brown sugar, Worcestershire sauce, mustard, chili powder, salt, and black pepper. Mix well to combine all ingredients.

Cover the slow cooker and cook on low for 4 to 6 hours or on high for 2 to 3 hours, stirring occasionally.

Once cooked, serve the sloppy joe mixture on hamburger buns.