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