In a COLD pan, add beef, and onion.
Turn heat to med-high.
Once beef is browned, add garlic.
Then add celery and green pepper.
Add mustard, cumin, ketchup, sugar, and season with salt and pepper.
Bring everything to boil, lower heat.
Then let it simmer and reduce.
It should be of a thick sticky consistency.
Serve on toasted burger buns.