Wash and soak millets in a container for 7-8 hours.
Wash and soak the urad dhal 20 minutes before you start grinding the batter.
First grind urad dhal with required water in a wet grinder to a soft and fluffy batter. t will take from 20 - 30 minutes to get the required fluffiness.
Transfer to a large container.
Now in the same grinder add the soaked millets with enough water and grind to a smooth batter.
Transfer the batter to the container with urad dhal batter.
Add enough salt and mix well.
Cover and let it ferment overnight. Make sure the container is large enough so that the batter can easily double in volume without spilling.
Beat the batter to remove gas and transfer to a container which is compact enough to hold the batter. This will go into the fridge and so there will be no further fermentation.
Heat a kadai and pour a ladle full of batter on it.
Slightly spread the batter to form a thick dosa.
Top it with onions and drizzle little oil on the sides.
Cook on medium flame until the base startes browning.
Flip the dosa and cook the other side too.
Serve hot with chutney of your choice.
