Wash all the ingredients and soak each seperately . Soak rice n sabudana for 4-5 hours . Urad dal and beten rice for 1-1.5 hours .
Grind the urad dal first into very very smooth paste adding enough water.Do not add water at once. Add it in intervals.
Once , urad dal is done , add rice, poha and sabudana and grind adding little water. Grind them all together into smooth batter . The final batter should be smoother than the normal idli batter.
Transfer the ground batter to a big vessel , add salt , mix well and keep aside for fermentation . let it ferment for 10-12 hours .
Morning the batter would have fermented . Mix gently before pouring into greased plates. Check the consistency . The batter should not be too watery nor too thick... just slightly thin.
Heat water in a steamer.If you are adding soda , add at this stage and mix. Grease idli plates with oil .Pour the batter into plates .
Steam for 10-12 mins on medium flame.Prick a tooth pick,if it comes clean it means its done.Allow it to cool a bit and remove from molds using a greased or wet spoon or knife.
Thats it soft, spongy Thatte Idli's are ready to serve. Serve with coconut Chutney / tomato chutney / Sambar / Gojju / Chutney Powder.
Notes
As said earier , typically cooked rice is used . So , if you are OK with it repalce sabudana with equal quantity of cooked rice .
I use small variety sabudana . So 4-5 hours soaking time enough. However , if you are using , bigger type, then you will have to soak them overnight . or the quickest way is add sabudana to hot water and keep it closed for 2-3 hours . This will reduce soaking time .
Adding cooking soda is optional. In hotels they use but i dont use it. You try and see which way you like.
The final batter should be smoother than the normal idli batter .
You can find thatte idli stand easily in steel vessel selling shops .