Dry roast the yellow moong dal till it starts turning slightly brown. You can skip this step but I really think it enhances the flavour of the idly a lot.
After it cools a bit, wash and soak along with the one cup brown rice. Let it soak overnight.
In the morning, grind it to a smooth batter and let it sit for an hour or so.
Heat oil in a small pan, add hing and roast for about 10 secs. Add the mustard seeds and let them splutter. Add curry leaves and red chillies. Fry for 1 minute and add it to the batter.
Add the eno to the batter and mix it combined.
Steam the idlies for 10-12 minutes and serve hot with chutney or sambhar.