2) Heat the oil in a large pan and add the mustard seeds. Wait for them to pop, then add the asafoetida, curry leaves, onions, chillies, turmeric and ginger. Allow to cook on a low heat for 5 minutes. Add the salt, sugar, lemon and potatoes. Toss together and add the chopped coriander.
Coconut and coriander chutney:
1) In a large bowl, soak the coconut in the coconut milk and set aside.
2) Meanwhile, blend together all other ingredients except the yogurt using a food processor or a pestle and mortar if you're feeling energetic.
3) Mix the blended ingredients with the soaked coconut and yogurt mixture.
1) Mix together the rice, urad dal, channa dal and fenugreek seeds and wash five to six times in cold water.
2) Place them in a large bowl and cover with warm water overnight or for at least 5 to 6 hours.
3) After the soaking period, discard the soaking water, reserving about 500ml.
4) Place the soaked rice, dals and methi seeds in a blender/grinder, and blend to a thick paste. The consistency of the batter should be just so that it covers the back of a spoon. Add a little bit of the reserved soaking water to help with the blending/grinding process. Blend/grind the batter to a degree where it's grainy - this is what's going to make the dosas crispy.
5) Place the dosa batter in an extra-large bowl, and put the bowl on top of a tray to catch the batter if the fermentation becomes too aggressive. Cover loosely and ferment overnight.
6) The next day, add salt, asafoetida and bicarbonate of soda, whisk lightly. It is important to add salt at this stage and NOT before as salt retards the fermentation process.
7) Lightly grease a flat nonstick pan with a halved onion and heat on a medium flame. Pour your batter into the middle of the pan with a ladle, making concentric circles outwards to spread thinly and evenly. Cook until the top becomes dry.
8) Spread a thin layer of oil on the top side of the pancake, toss and cook for a couple of seconds on the other side.
9) Spread the pancake with Coconut and coriander chutney, fill with the potato mixture, roll and serve immediately.