Heat a large skillet over medium heat. Add the ground beef and chopped onion and cook, stirring, until the beef is no longer pink and the onion is soft. Drain off excess grease. In a small bowl, combine the brown sugar, mustard, barbecue sauce, and ketchup until blended (the brown sugar should be starting to dissolve). Add the barbecue mixture to the skillet and stir well to combine. Let the beef simmer, stirring as needed, for 5 minutes or until the sauce is heated through and thickened. Serve the sloppy joe sauce on buns with any additional condiments as desired.