To prepare Rava Idly quickly you can make ready made mix in your house and keep them in the fridge. Whenever you want to prepare you can remove the mix from the fridge and make idly as per your wish.

Take Rava and fry well, keep aside. Pour oil in a pan and put Mustard seeds, Udad dal, Channa dal, Cashew Nut, Turmeric powder & Green Chilies.
After Mustard Seeds bursts, add the above one by one.

Make the stove simmer and fry Green Chilies till they turn brown.
Now add fried Rava, Turmeric Powder and salt. Fry them well.
Rava should fry well in the oil. Remove from the fire and let it cool to room temperature.
Now add Asafetida. Mix wee and keep the Ready Made Mix in the fridge.

When ever you want to prepare Rava Idly, take a bowl, pour buttermilk or water in the bowl. Add the ready made mix into that according to your need. Mix well and keep aside for 1 hour. Then make Idly asusual.

Serve hot with any chutney or pickle.