Soak the tamarind in water for 10 -15 minutes. Squeeze the extract from tamarind and set aside. Discard the pulp.
Heat oil in a vessel and add the mustard seeds and chana dal. Once they start to splutter, add the fenugreek seeds, curry leaves and hing and saute for a couple of seconds.
Add the chopped onions and fry until they are transparent.
Add the chopped sweet potatoes and turmeric to it and saute for a minute.
Add the sambar powder, 2 cups of tamarind extract, jaggery and salt.
Dilute the rice flour in little water and add it to the sweet potato mixture.
Cover it with a lid and let it cook on a medium flame until the pieces are soft.
When the gravy thickens, remove it from the flame.