Soak the rice and urad dal in 2 separate bowls of water for 6 hours.
Drain the rice, transfer to a blender or food processor and process, gradually adding 375ml / 13fl oz (1 1/2 cups) water to make a coarse batter. Transfer to a bowl.
Drain the urad dal, transfer to a blender or food processor and process, gradually adding 250ml / 8fl oz (1 cup) water to make a smooth batter.
Combine both batters, season with salt and set aside to ferment for 10 hours.
The batter should be thicker than dosai batter, but of a pouring consistency, so add a little more water if necessary.
An hour before cooking, soak the chana dal in a bowl of water.
Drain well and add to the batter, then stir in the coconut, ginger, chillies and coriander leaves.
Heat the 2 teaspoons oil for tempering in a small, heavy-based frying pan (skillet).
Add the mustard and cumin seeds, asafoetida and curry leaves and stir-fry for 1 minute, or until the mustard seeds start spluttering.
Stir the contents of the pan into the batter.
Oil idli moulds with a little oil and pour 4 tablespoons of the batter into them.
Steam in a steamer or pressure cooker for about 15 - 20 minutes, or until cooked.
To check if the idlis are done, insert a cocktail stick (toothpick) into the middle and if it comes out clean, they are cooked.
If using a pressure cooker, don't apply the weight or whistle. Carefully remove from the steamer and allow to cool for 1 - 2 minutes.
If you don’t have idli moulds then use small heatproof bowls, such as ramekins.
Oil them lightly before filling them with the batter and steaming them in either a double boiler or an egg poacher.