Brown meat in a pan in 1 tbsp coconut oil, then put a lid on the pan and cook until just barely cooked through about 3-4 minutes. Set aside.
Fry onion in coconut oil until just browned. Add in the garlic, salt, and dry spices and cook for another 1-2 minutes. Add vinegar to deglaze the pan, then add in cooked lamb, mustard and coconut milk. If using, add diced boiled potato cubes at this point.
Stir to combine and remove from heat. Serve with rice and naan.