Heat the ghee in a frying pan (skillet).
Add the dals and mustard seeds and fry for about 1 minute, or until they turn dark brown.
Transfer to a large bowl and set aside.
Heat the oil in a large pan, add the onions, cashew nuts and chillies, and stir-fry for about 2 minutes.
Add the ginger paste, vermicelli and semolina and stir-fry for a further 2 minutes.
Remove from the heat and slowly add the yoghurt, bicarbonate of soda, coriander, lime juice and season with salt.
Mix well, then add this mixture to the dals and set aside for about 30 minutes.
Oil idli moulds with a little oil and pour a ladleful of the mixture into them.
Steam in a steamer or pressure cooker for about 8-10 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 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.