Soak urad dal for 2 hour at least. (Use whole urad dal without skin for better result). Grind urad dal with only little water till fluffy (Add urad dal little by little(don't add water)while the grinder is running, just sprinkle water now and then).
Add less water while griding, else it will consume more oil. Now add onions,ginger,green chillies,curry leaves, rice flour in the batter along with salt and asafetida. Mix well and keep it aside. Meanwhile heat the oil in a kadai.
Wet your right hand, take a small amount of batter and make a hole with your thumb in the middle of the taken batter. Deep Fry in medium flame,turn the vadais' and fry till both the sides turn crisp and golden brown.