In a large skillet, cook the ground beef and chopped onion over medium heat until beef is browned and onion is tender, about 7 minutes. Drain excess fat.
Stir in tomato sauce, ketchup, brown sugar, and yellow mustard. Reduce heat and simmer for 10 minutes, stirring occasionally.
Serve the sloppy joe mixture on toasted hamburger buns.
Notes
This recipe is perfect for a quick weeknight dinner and can be doubled for larger gatherings.