In a bowl, add 1 cup fine rava and 1 cup curd or yogurt and salt. it is best to use slightly sour curd. Whisk the batter for 3-4 minutes, so air particles incorporate into the batter.
Cover and rest the batter for 20 minutes.
For making potato stuffing, in a pan, add oil, mustard seeds, urad dal, chana dal, curry leaves, 2 chopped green chilli, 1 tsp ginger paste, 1 medium size onion slice, turmeric powder and boiled and mashed potatoes. Mix well.
Switch off the gas, and add lemon juice and coriander leaves. mix well. Potato stuffing is ready, make a small tikki from the stuffing.
Now, after 30 minutes add ½ tsp of baking soda, and lemon juice and give a stir. you can now see the batter turning slightly frothy. do not mix more.
grease the idli plates and switch on the steamer with enough water.
pour 1 tbsp of idli batter into each impression. place the flattened potato stuffing
and gently press the aloo stuffing. pour again 1 tbsp idli batter to cover the filling.
stack up all the idli plates and steam for 12-15 mins. rest it for another 3-4 mins.
unmould the idli plate and serve hot with chutney.