Wash the urad dal 2-3 times. Soak in water with methi seeds for 5-6 hours.
In a separate container soak the idli rava or idli rice for 5-6 hours.
Soak Poha 10-15 minutes before starting to grind the rice and dal.
Drain the water of urad dal. add soaked poha or cooked rice and grind until the dal becomes smooth and puffy paste. Not too thick, not too thin paste. Add little water while grinding if required. You may use the dal-methi soaked water.
Transfer the batter to a container.
Now drain the water from idli rava / rice and grind it to smooth but thick paste using little wate. Add this to the dal paste container. Mix well and cover the container.
Let it ferment in room temperature for another 5-6 hours or overnight. The batter will increase in volume.
Add salt and soda. Whisk the batter well for 3 minutes.
Heat water in an idly vessel and put it on high flame. Grease the idly plates with little oil and gently fill the rounds with the batter. Put it inside the idly vessel and let it steam for 15-20 minutes. Do not overcook, otherwise it might become hard and dry. Just keep an eye and remove the idlis once not sticky.
Take out the idly plates and leave it for 4-5 minutes. Do not remove the idlis immediately.
Take out the idlis with a spoon or knife and serve hot with chutney and sambar.