Heat a large skillet over medium heat. Add the ground beef and cook, breaking it apart with a wooden spoon, until browned, about 5 minutes.

Stir in the chopped onion and sauté until translucent, about 3-4 minutes.

Drain any excess fat from the skillet to reduce greasiness.

Add ketchup, brown sugar, Worcestershire sauce, and mustard; stir to combine.

Season with salt and pepper to taste.

Reduce heat to low and simmer the mixture for 10 minutes, stirring occasionally.

Meanwhile, toast the hamburger buns under a broiler or in a toaster until lightly golden, about 1-2 minutes.

Spoon the sloppy joe mixture onto the toasted buns and serve immediately.