Take the black gram lentils (urad dal) in a bowl and rinse it well with enough water. Soak it for about 2-3 hours. Take idli rava in another bowl and soak it with enough water for about 20 minutes.
Drain all the water from soaked lentils and transfer it to mixer. Add half cup of water and grind to fine and fluffy batter.
Transfer it to a vessel. Next, squeeze water from soaked idli rava and add it to lentil batter. Mix them together.
Keep the batter covered for fermentation for about 8 hours. Next add salt to the fermented batter. And Keep it ready.
Next for stuffing, heat a pan with oil, mustard seeds, cumin seeds and lentils. Saute till they splutter. Next add curry leaves, green chillies and onion. Continue sauteing till onion turns translucent.
Add turmeric powder, cooked potatoes, salt and mix well. Finally add coriander leaves and squeeze lemon over it. Stuffing is ready now. Take the idli plate, pour little idli batter in to each depression. Add a tbsp of stuffing at the center of each.
Again cover the stuffing with idli batter. Keep the idli plate in steamer and steam it for 10 minutes. Remove the idli and repeat the process with rest of the batter.
Stuffed idli is ready to be served with coconut chutney and sambar.