Heat a pan with oil. Add mustard seeds and let it splutter.
Add urad dal, asafoetida, broken dry red chilies and cook for few seconds till the dal becomes golden brown.
Add onions, green chilies, curry leaves and saute till the onions become translucent.
Add tamarind extract, red chili powder, salt and mix well. Cook for 1 minute over low flame.
Add cooked mochai (butter beans), grated coconut, mix well and cook for 1 more minute.
Switch off stove and close the pan tightly with a lid and let it rest for 5 to 7 mins for the mochai to absorb all the flavors.
Mochai sundal is ready. Serve hot or warm.
This stays well for 3 to 4 days when kept refrigerated.