Brown the ground beef in a large skillet over medium-high heat. Drain off any excess grease.
Add the onion and cook until softened, about 5 minutes.
Stir in the ketchup, brown sugar, Worcestershire sauce, mustard, garlic powder, salt, and pepper. Bring to a simmer and cook for 10 minutes, or until the sauce has thickened slightly.
Serve the Sloppy Joe mixture on hamburger buns.
Notes
For a spicier Sloppy Joe, add a pinch of cayenne pepper to the sauce.