In a large fry pan, over medium-high heat, cook the ground beef. Drain the excess fat from the ground beef once it is fully cooked and return the ground beef to the pan.
Into the fry pan, add the red onion, ketchup, mustard, brown sugar, cumin, red pepper flakes, and salt. Stir to combine with the ground beef. Cook over medium heat for an additional 5 minutes. Remove from heat and set aside.