Soak rava in whisked yogurt for 5 mins. Add oats to it and keep for another 5 minutes.
Heat some oil and add mustard seeds, urad dal, chana dal, chopped onions, green chilli and fry till the onions become soft, no need to become brown. Add this to the oats and rava mixture. Add salt, coriander leaves, bicarbonate of soda and mix well. Add water (refer notes) to adjust the consistency of the batter.
Grease the idli moulds and out a browned cashew (if using) and some grated carrots at the base. Pour the rava and oats mixture over it. Steam cook for 7 mins (till the steam comes from cooker vent).
Serve hot with chutney and sambar.
Notes
I made only half the qty of the above recipe. I got 10 idli's from half the qty. I used 1/4 tsp of bicarbonate of soda for half the qty also. I used 1/2 - 3/4 cup of water for half the qty.You can add other veggies like cooked peas also in this recipe. Add it along with the cooked onions.It's better to serve it immediately.