Heat the oil in a large pan, add the lentils and semolina and fry for about 2 minutes, or until lightly roasted.
Put the yoghurt, baking powder, ginger paste, curry leaves, asafoetida, chillies, mustard seeds and 250ml / 8fl oz (1 cup) water in a large bowl, season with salt and mix together.
Set aside for 30 minutes, then whisk the mixture to make a batter. Set aside for 4-6 hours.
Oil idli moulds with a little oil and pour a ladleful of the batter into them.
Steam in a steamer or pressure cooker for about 10-12 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 heat proof 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.