Soak tamarind in about 1 cup of water for about 5 minutes. Extract its pulp. Add another cup of water and extract as much as tamarind juice as possible. Discard the remaining vein of the tamarind. Keep this tamarind juice ready.
Heat 2 tbsp oil in a kadai and add the mustard seeds. When it crackles, add the fenugreek seeds and channa dal.
Once the dal turns golden brown, add the asafoetida, dry red chillies and curry leaves.
Add the manathakkali vathal and fry for about half a minute.
Add the sambar powder and fry for another half a minute.
Now add the extracted tamarind juice and salt.
Cook on medium flame for about 15 minutes on medium flame. Reduce the flame to low and cook for another 15 minutes.
By now, the gravy would have thickened and oil would have separated. The gravy will thicken as it cools so maintain the gravy consistency accordingly.
Finally add the remaining 1 tbsp oil and some fresh curry leaves. Switch off.