Wash millets and urad dal 2-3 times thoroughly until the water is no longer turbid/starchy. Add thick poha and soak them in enough water for 4 hours at the least.
Blend the soaked millets and dal into a smooth batter adding as little as possible as needed. The batter should be thick. If using a mixie/mixer grinder, grind the batter in batches with ice cold water. It is important to not let the batter heat up by running the mixer continuously.
Remove the batter onto a large container and add salt required (I usually add a teaspoon of fine salt). Using your hand, mix the batter well. Keep it in a warm place for another 8 hours/leave it overnight untouched. The batter should be doubled up in quantity. Beat it down and store it in the fridge until required. The batter should be neither be too thick nor too runny.
To make Millet Idli -
Add some water to the fermented millet idli batter if required, mix well. The batter should be thick and pourable but not too runny. Fill the idli mould with the prepared idli batter.
Add 2 cups of water in the idli steamer/cooker pot and let it come to a boil on high flame. Set the idli plates on the steamer.
Steam the idlis for 10-12 minutes on medium flame.
When the idli steamer is opened, idlis should have risen up and look cooked. Switch off the heat and let the idlis stay in the steamer for another 5-mins. Remove the idli plates from the steamer and sprinkle some cold water all over the idlis. Wait for another 5 mins and gently remove the idlis from the mould.
Serve hot with coconut chutney, sambar and idli podi.