1. Grind daal mixture together to a very smooth consistency. Add salt and enough water to make into a dropping consistency. Leave to ferment for about 5-6 hours, till a little spongy.
2. If thickened too much, add a little water to thin a bit. Heat tawa, and brush oil over it. When really hot, splash a little water over it, and immediately pour batter onto it, spreading it thin, with a circular motion. This will have to be very swift and will need a bit of practice.
3. After spreading the batter, lower the heat and dribble a little oil around the edges so that it seeps under the dosa. When edges start browning a bit, pass a flat spoon under it to ease the dosa off the pan.
4. Put desired filling in the centre and fold the two edges over. Serve accompanied with sambhar and chutney
For the Masala Filling
1. Heat the oil in a heavy based pan and add the mustard seeds, then onions, curry leaves and green chillies, and sauté over high heat till the onions are a little transparent.
2. Add the salt and the turmeric and mix well before adding the potatoes. Turn the potatoes around till well mixed and add the water and let it simmer for 2-3 minutes.