Roast semolina (rava) lightly in a pan, then mix it with yoghurt, water and salt to form a smooth batter.
Boil and mash potatoes, then add them to the batter for a soft texture and extra flavour.
Temper mustard seeds, cumin, curry leaves and green chillies in oil, then mix into the batter for a burst of taste.
Grease idli moulds and pour the batter into them. Steam for 10-12 minutes until cooked through.
Remove the idlis from the moulds and serve hot with coconut chutney and sambar for a complete meal.