Grind the black gram with a little water to a smooth paste. Add the semolina to the black gram paste and mix well.
Add the ragi flour to the semolina-black gram mixture and combine everything together. Cover the bowl and keep it in a warm place to ferment for 8 hours.
Heat oil in a non-stick pan, add mustard seeds and wait till they splutter. Add curry leaves and saute for a few seconds.
Add this tempering to the fermented batter along with salt and baking soda and mix well.
Heat sufficient water in a steamer. Grease idli moulds with some oil. Pour a ladleful of batter in each mould, place in the steamer, cover and steam for 12-13 minutes.
Wait for the idlis to cool before removing them from their moulds.
