Wash and soak 1 cup urad dal in a bowl with 2 cups of water overnight.
Wash and soak rice and all the other ingredients except salt in 6 cups of water overnight.
Blend rice mix in a mixer with little water,once its nearly done, add the urad dal without any water.
You may blend the above in 2 or more batches depending on the capacity of the mixer.
Mix salt in the batter.
Keep the batter aside to ferment overnight in a warm place.
Heat the cast iron tawa for few mins on low heat.
In a bowl take some amount of batter and add some water to get a thin pouring consistency.You may add salt if required
Once its medium hot,add a ladle of batter and spread it evenly.Cover it with a lid
Let it cook for few minutes ,until rawness dissapear.Flip it and cook on the other side for few mins.
Using a spatula,take out the dosa.
Serve hot with coconut chutney
Before you start with the next uttapam, add few drops of water on the tava, so that the temperature of the tava has come down,else the batter will stick.