Cut tomatoes into big pieces.
In a kadai put one teaspoon oil and fry Urad dhal till it turns slightly red. Add green chillies and fry for a while. Then add cut tomatoes and asafotida powder and fry for two to three minutes on a low flame. before removing add coriander leaves and fry just for a second. Switch off the stove. Allow to cool. Add a teaspoon of salt and tamarind and grind altogether to a paste.
Season it with mustard seeds.(optional)