Pressure cook the potatoes till done. Peel and roughly chop.
In a saucepan, heat the ghee. Splutter the mustard seeds and fry the urad dal.
Fry the ginger and green chillies. Add the cashew nuts and just toss.
Immediately add the onion and cook just till wilted.
Add the chopped tomato, turmeric powder and salt and give a good toss. Mash the mixture as you go. Sprinkle in little water if it sticks.
Cook for five minutes. Add the coriander leaves and switch off.
Make dosa as usual. Spread a couple of tablespoons of the potato mixture in the middle and fold.
Serve hot with sambar and chutney.