in a large mixing bowl take ½ cup sabudana / sago / javvarisi / sabbakki. wash them if they are not cleannext, add 1 cup of idli rava. rinse them well, if they are not clean.now add 2 cups of sour curd. optionally you can use fresh curd, however, overnight fermentation will help curd to turn sour.depending on the curd thickness you may have to add some water to make batter watery. make the batter slightly thin as the sago and rava will absorb water during fermentation. add around 1 cup of water if you are using thick curd.keep the batter to rest and ferment overnight or atleast 8 hours. during this time, sago will soak well and turn to nice pearls.mix the batter well. do not mash as the pearls of sabudana may break.now add ¼-½ cup water as required to adjust the consistency of idli batter.add some salt to taste.just before steaming add a pinch of baking soda and mix well till it turns frothy.brush the plates with oilplace cashew in the mould.and pour the batter immediately into the idli plate. do not rest the batter.steam it for 8-10 mins on medium flame. allow it to rest for 5 mins then unmould.serve hot with any chutney of your choice.