Soak tamarind in vinegar for an hour to get a thick pulp.Heat mustard oil in pan and ginger,garlic slices and saute for 2 min.Add turmeric slices and saute for another 2 min till they are light brown only.Add tamarind pulp ,cumin seed ,sugar and cook it for 2 minute.Add chopped chili, mustard powder ,salt and cook for about 8-10 minute at medium flame till you get a thick gravy consistency .Switch off the flame and let it rest for minimum an hour.Even you can keep this for a day or two at this stage.