Preheat oven to 175C.
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, then saute.
Add mustard, cumin, ketchup, sugar, and season with salt and pepper.
Bring everything to boil, lower heat.
Then let it simmer to reduce and thicken sauce.
Layer baking dish with sloppy joe.
Top with mashed potatoes.
Then sprinkle cheese on to of mashed potatoes.
Bake for 20-30 minutes until cheese nicely melted.