Cook the ground beef with chopped onion and bell pepper in a large skillet over medium heat until the meat is browned and vegetables are tender; drain any excess fat.

Stir in the tomato sauce, ketchup, brown sugar, Worcestershire sauce, and mustard powder. Bring the mixture to a simmer and cook for 10-15 minutes, allowing it to thicken slightly.

Spoon the sloppy joe mixture onto hamburger buns and serve warm.