Soak the rice and lentils in 2 separate large bowls of water for 1 hour.
Drain the rice.
Transfer to a blender or foodprocessor and process, gradually adding about 125ml / 4 1/2 fl oz (1/2 cup) water to make a coarse batter. Transfer to a bowl.
Drain the lentils.
Transfer to a blender or food processor and process until almost smooth but still a little grainy.
Add the pressed rice, coconut, chillies, ginger and coriander leaves and process, gradually adding 4 tablespoons water to make a smooth batter.
Combine both batters, season with salt and set aside to ferment for about 4 hours.
The batter should be of a thick pouring consistency, so add a little more water, if necessary.
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 20 minutes, or until cooked.
To check if the idlis are done, insert a cocktail stick (toothpick) into the middle and 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.