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 sliced onions and fry until they are transparent.
Add turmeric, sambar powder, 2 cups of tamarind extract, jaggery and salt to it.
Cover and cook over a medium flame until the onions are soft.