Heat Oil in pan. Add methi daana, urad daal and let it splutter.Do this on medium low flame and we do not want our daal to turn black.
Add the chopped onions and saute till golden brown.Add the tomatoes and red chillies and cook till tomatoes are soft and mushy.
Switch off the flame and let this mixture cool down.
Once its cool down, grind this mixture along with tamarind paste till very fine.First add 1/2 tsp tamarind paste and check for sourness and then add the rest if needed.Add water gradually till the right consistency achieved.