Soak 1 cup of moong dal in water for about 2 hours.
Drain the water and grind the moong dal in a mixer to make a coarse paste. Do not add water while grinding.
Add 1 finely chopped onion, 2 finely chopped green chillies, a handful of chopped coriander leaves, 1 teaspoon of cumin seeds, a pinch of asafoetida, and salt to taste to the paste. Mix well.
Heat oil in a kadai for deep frying.
Take a small portion of the mixture, flatten it slightly and drop it in the hot oil. Fry on medium heat until it turns golden brown.
Drain the vadas on a paper towel to remove excess oil.
Serve hot with ketchup or any chutney of your choice.