In a large skillet over medium-high heat, brown the ground beef with diced onion, breaking it up with a spoon as it cooks, about 7-8 minutes. Drain excess fat if needed.
Add ketchup, brown sugar, Worcestershire sauce, mustard, and garlic powder to the skillet. Stir well to combine.
Reduce heat to low and simmer for 10-12 minutes, stirring occasionally, until the sauce thickens. Season with salt and pepper to taste.
Toast the hamburger buns if desired, then spoon the sloppy joe mixture generously onto each bun. Serve immediately with chips or a simple side salad.