Wash whole green gram, rice and soak for 8 hours or overnight in plenty of water.
In the morning discard the water used for soaking and transfer the ingredients to a food processor.
Add green chilli, ginger, salt and grind to a smooth consistency using required amount of water.
You will need about 1/4 – 1/2 cup of water. The batter should be thin. Transfer to a bowl.
Chop the onion finely and add it along with cumin seeds and asafetida to the batter.
Let the batter rest for 45 min to 1 hour. (This step is purely optional).
Heat a tawa and grease the surface with oil.
Pour a ladle of batter on the tawa and spread it evenly around to form a 6 to 7 inch circular shape.
Drizzle 1/4 tsp – 1/2 tsp of oil around the dosa and let it cook over medium flame.
When done, flip it over and let it cook on the other side too till it is evenly browned.
Fold and transfer to a serving plate and continue till all the batter is over.
Serve this Pesarattu dosa piping hot with ginger or coconut chutney.