- Prepare the masala filling: Heat the oil in a heavy based pan and add the mustard seeds, then onions, curry leaves and green chillies, and saute over high heat till the onions are a little transparent. 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. Wash and soak the rice in one container and the dal and fenugreek seeds together in another container for 5-6 hours or over night, depending on the weather. Grind dal mixture together to a very smooth consistency. Next grind the rice smooth too and mix the two batters. Add salt and enough water to make into a dropping consistency. Leave to ferment over-night. Heat tawa, and brush oil over it. When hot pour batter onto it, spreading it thin, with a circular motion. 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. Put filling in the center, and fold and serve hot.