Wash horse gram thoroughly and soak it in water for about 4 hours. In another bowl, wash idli rice, urad dal, and fenugreek seeds and soak them separately for 4 hours.
Drain the soaked horse gram and grind it in a mixer grinder using a little cold water until smooth. Transfer the ground batter to a bowl.
Grind the soaked idli rice, cooked rice, urad dal, and fenugreek seeds in a mixer grinder, adding a little water gradually until a smooth batter forms.
Add the rice batter to the ground horse gram batter. Add cooked rice and salt, then mix everything well using your hands until the batter becomes smooth.
Cover the batter and allow it to ferment for about 8 hours or overnight until it becomes light and airy.
Pour the fermented batter into greased idli moulds and steam for 10–12 minutes until soft and cooked.
Remove the soft kollu idlis from the mould and serve warm with chutney or sambar.
