Heat a large skillet over medium-high heat. Add the ground beef and cook for 4–5 minutes, breaking it into small crumbles.
Add the onion and green bell pepper. Cook for another 3–4 minutes, until the vegetables begin to soften and the beef is fully browned. Drain excess grease if needed.
Reduce the heat to medium. Add the garlic and cook for 30–60 seconds, until fragrant.
Stir in the ketchup, water or broth, mustard, brown sugar, Worcestershire sauce, and chili powder. Season with salt and black pepper.
Reduce the heat to low and simmer uncovered for 8–10 minutes, stirring occasionally, until the sauce thickens and coats the beef.
Toast the hamburger buns in a separate skillet until lightly golden, using butter if desired.
Spoon the beef mixture onto the buns and serve hot.
