Heat a large skillet over medium heat and add the ground beef. Break it up with a wooden spoon and cook until browned and crumbly, about 6-8 minutes. Drain excess grease if necessary.

Add the chopped onion, garlic, and green bell pepper to the skillet. Cook for 4 minutes, stirring occasionally, until vegetables are soft and fragrant.

Stir in the ketchup, tomato paste, yellow mustard, brown sugar, Worcestershire sauce, salt, and black pepper. Mix well until ingredients are evenly combined.

Reduce heat to low and simmer the mixture for at least 10 minutes, stirring frequently. Let the sauce thicken and coat the meat completely. If the mixture is too thick, add a splash of water and stir.

Scoop the sloppy joe mixture generously onto hamburger buns. Toast buns beforehand for extra flavor if desired. Serve hot.