Soak the Urad dal with methi seeds overnight or for a min of 6 hours in water (which is about 1.5 inches above the top)
Drain the water and retain. Wash and soak the Poha in water for 1 hour before grinding.
Soak Idli Rava in that drained water adding 1 cup more water.
Start grinding the Urad dal (with the soaked methi seeds) to a smooth paste, Add the soaked poha and keep grinding for 20 mins ( I use a stone grinder, and I run the Urad to a real smooth paste). If using Mixer grinder, keep adding 1 tsp of cold water at a time and grind the Urad and Poha to a smooth batter.
Now take 2 fistfuls of the Soaked Idli Rava and add to the batter which is still being ground. after 3 mins, remove the Urad+poha batter to a large bowl / steel box. Keep squeezing the Idli Rava and add all the soaked and squeezed out Idli Rava to the ground batter. Add salt and mix well with your hands for a good 8-9 mins (the biceps are loving this workout).
Cover and ferment overnight or for a good 8-10 hours.
Fluff up the fermented batter well. ,Grease the Idli plates (see pic of the special thatte Idli plates - you can also use a Dhokla steaming plate, but making it too large will not give enough height / thickness to the Idlis)
Pour 2-3 ladlefuls of the batter to the greased plates and place it in the stand, if using. Repeat for all plates.
Steam for 18-22 mins until a knife comes out clean and dry. Cool for 2-3 mins.
Unmould and serve hot with Darshini Sambhar / Tamilnadu styleTiffin Sambhar and/ or Chutney / Molagapudi