In a large skillet over medium-high heat, brown the ground beef until no longer pink.

Drain the excess fat from the skillet and transfer the cooked beef to the slow cooker insert.

Add the diced onion, green bell pepper, and minced garlic to the slow cooker.

In a separate bowl, whisk together the ketchup, brown sugar, mustard, Worcestershire sauce, chili powder, salt, and pepper.

Pour the sauce mixture over the beef and vegetables in the slow cooker and stir to combine thoroughly.

Cover and cook on Low for 4 hours (or High for 2 hours) until the vegetables are tender and the sauce has thickened.

Stir once more before serving and spoon the mixture onto toasted hamburger buns.