Soak the basmati rice and parboiled rice in a large bowl of water for 6 hours.
At the same time, soak both lentils and the fenugreek seeds in another large bowl of water for 6 hours.
Drain the rice and transfer to a blender or food processor and process, gradually adding 250ml / 8fl oz (1 cup) water, to make a smooth batter.
Drain the lentils, put into a clean bowl and mix in the pressed rice.
Transfer to a blender or food processor, and process, gradually adding 250ml / 8fl oz (1 cup) water, until light and fluffy.
Combine both batters in a large, clean bowl, season with salt and set aside to ferment for 8-10 hours.
When ready to cook the dosas, add a little water, if necessary, to make a thick batter of pouring consistency.
For the filling, cook the potatoes in a pan of boiling water for about 20 minutes, or until soft. Drain and allow to cool.
When cool, peel off the skin, return to the pan, and mash.
Cook the peas in a separate pan of boiling water, for 5-7 minutes, or until soft. Drain and set aside.
Heat the oil for tempering in a heavy-based pan, add the remaining ingredients for the tempering, and stir-fry for 1 minute, or until the mustard seeds start spluttering.
Add the onions, ginger and chillies for the filling and stir-fry for a further 2 minutes.
Stir in the mint, turmeric, salt, tomato, and 250ml / 8fl oz (1 cup) water, then cover the pan and simmer for about 5 minutes, or until the onions and tomato are soft. Mix in the potatoes and peas.
The filling should be dry, so if there is too much water, cook, uncovered, over low heat until all the moisture has evaporated.
Remove from the heat and stir in the lime juice.
Heat a tawa, flat griddle or frying pan (skillet) over high heat until it sizzles when a few drops of water are sprinkled on it.
Reduce the heat and smear a very thin film of oil evenly over the Tawa. Pour a ladleful of batter into the center of Tawa and spread it out quickly using a circular motion with the back of the ladle to form a 15cm / 6 inch round dosa.
Drizzle 1 teaspoon oil around the edges, then increase the heat and cook for about 2 minutes, or until the base is golden brown.
Turn the dosa carefully and fry for a further 2 minutes, or until golden brown.
Transfer to a serving plate, place a little of the filling in the middle, and fold over. Serve hot with a little Coconut Chutney smeared over the top and some butter. Make the others in the same way.