Mix sooji, buttermilk, and salt. Soak for 30 minutes.
For tadka, heat oil in a small frying pan. Add urad dal, chana dal, mustard seeds, and curry leaves. Then add onions and green chillis. Fry till onions are golden brown. Cool the onions slightly and add to the sooji rava batter. Add coriander leaves to the batter. Add the soda bicarb to the batter just before steaming.
Grease idli plates. Add batter to the wells and steam 10 – 15 minutes till idlis are cooked. Serve immediately.
Note: You can add any vegetables you like to the rava idlis. Grated carrots, chopped tomatoes, green peas, grated coconut all work well.