Heat oil in a pan.
Add mustard seeds to it and let it sputter for a while.
Add cumin seeds and curry leaves.
Now sprinkle asafoetida in it.
Then add onion and saute till it becomes golden brown in color.
Then add tomatoes to it.

Add all the above mention spices to it.
Mix it well.
Add water to it and stir continuously the gravy.
Check it out that gravy should be thick.
As soon as oil comes out from the gravy.
Switch off the gas.

Let this gravy totally cool down on room temperature.
After that finally add dahi to it and mix it well.
Garnish with coriander leaves.
Eat this delicious sabji with roti,paratha or wheat bhakri.