Soak moong and urad dal, separately, overnight. In the morning, grind them into smooth batter with green chilies, cumin, ginger and salt.
Heat about 1½ to 2 cups of oil (I used canola oil) on medium-high flame. When the oil is hot enough, flatten a lemon size batter either on your palm or on a plastic Ziploc bag, make a small hole in the middle, slowly slide the batter into the hot oil. Fry the vadas on medium flame on both sides, till slightly golden brown.
Serve hot with any chutney (tomato, coriander, ginger, mint or good old coconut).