Soak the rice and urad dal in 2 large bowls of water overnight.
Drain the rice and lentils separately, then transfer the rice to a blender or food processor and process until ground.
Transfer to a clean bowl.
Do the same with the lentils, then add the ground rice and mix together.
Season with salt, cover with a damp cloth, and allow to ferment overnight.
The next day, cook the potatoes in a pan of boiling water for about 15 minutes, or until soft, then drain and allow to cool.
When the potatoes are cool, peel off their skins, return to the pan and mash.
To prepare the filling, heat the oil in a pan add mustard seeds, chana dal and curry leaves and stir-fry for about 3 minutes.
Add the onion and fry for about 2 minutes, then add the chilli powder and turmeric and cook for a further 2-3 minutes.
Finally, add the mashed potatoes and stir to combine.
When you are ready to cook the dosas, add enough water to the rice and lentil paste to make a batter of thick pouring consistency.
Heat a large, non-stick flat tawa, pan or a cast-iron griddle and brush it lightly it with a little ghee or oil.
Pour in 2 tablespoons of the dosa batter and quickly spread it evenly into a circle of about 15cm / 6 inches diameter.
Flatten the surface with the back of a ladle then pour a little oil around the edges of the dosa so it does not stick to the pan.
Cover the pan for 1 minute, or until crisp and golden, then flip it over to cook the other side.
Transfer to a serving plate, place some of the filling in the centre and fold thedosa in half. The dosas must be served hot with Coconut Chutney and piping hot Sambhar.
Make the others in the same way.