Heat Oil in a pan. On medium heat, fry urad dal, chana dal, cumin seeds, asafoetida, cashews and curry leaves till the lentils and cashews turn slightly brown.
Now add semolina and roast, and mix well. Now take out the mixture and let it cool.
Grind the soybeans roughly.
Once the semolina mixture is cooled, add the ground soybeans and mix well.
Add yoghurt and salt and mix everything. Cover and let the mixture rest for about 10 minutes.
After 10 minutes, you will see that the mixture has absorbed all the moisture and has become thick. Add water and adjust the consistency. The batter should neither be too thick nor runny.
Meanwhile, heat the idli steamer until the water bubbles. Grease the idli trays with oil or ghee. Add baking soda to the batter. Mix well and pour portions of the idli batter, filling up to 3/4s of the level. Place the plates in the steamer ONLY after the water starts bubbling.
Now steam the idlis for about 15 minutes on high heat. After 15 minutes, take it off the heat and let it rest, with the lid closed, in the steamer for another 5 minutes. Then take them out.