Firstly, rinse ½ cup whole moong beans & 2 tbsp rice a couple of times in water. Then soak the moong beans & rice for 4 to 6 hours or overnight in enough water. Drain all the water.
In a blender or mixer-grinder, add the moong beans and rice after draining all the water. You can even rinse the moong beans and rice before grinding.
Add 1 green chilli (chopped), 2 tbsp chopped coriander leaves, 1 inch ginger (chopped), ½ tsp cumin seeds, 1 pinch asafoetida (optional), salt as per taste and ⅓ to ½ cup water.
Grind or blend to a smooth batter.
On a cast iron tawa or skillet , smear some oil or ghee with a paper towel or with half of an onion. Keep the heat to low or medium-low when spreading the pesarattu batter.
With a big spoon, pour the pesarattu batter on the griddle and use the same spoon for spreading the batter into a round shape
Sprinkle some finely chopped onions, chopped green chilies (optional) and coriander leaves (optional).
Flip & cook the second side for a half a minute or a minute. This will just lightly cook the onions.
Fold and serve moong dal dosa.
Repeat with the rest of the batter. Serve warm.