Wash thuvar dhal and add two cups of water along with a pinch of turmeric powder and pressure cook till soft.
Soak tamarind in water and squeeze out the juice.
Cut tomato into medium size pieces. Remove the skin from radish and cut it into thin round pieces. Add water just to cover the radish pieces. Add tomato pieces, salt, sambar powder, turmeric powder and cook on medium flame till the radish is soft. Add tamarind juice and allow to boil. Now add the cooked thuvar dhal mashed well. In a kadai put the oil and when it is hot add the mustard. When it pops up, add asafotida, cut onions and curry leaves and fry for a while. Add this to the sambar. Mix well. Remove from stove.