Heat oil. Add mustard seeds, fenugreek, channa dal or toor dal, curry leaves. When mustard seeds crackle and dal turns brown, add sliced onion and fry till it turns soft. Soak tamarind in warm water for 10 minutes and extract 3.5 cups of juice from the same. Add this juice to the cooked onion.
Add salt, sambar powder, hing and allow the mixture to boil for 20 minutes or until oil starts to float on top.
Add jaggery if you like the gravy to be slightly sweet.
Mix rice flour in 1 tsp water and make a thick paste.
Add this paste to the tamarind gravy and stir continuously until the mixture becomes little thick.