First we will prepare the Masala :
Heat oil in a heavy based pan, put the mustard seeds and let them crack. Afterwards put the chopped ginger,garlic and chilly. Saute, then add onions, curry leaves, turmeric powder. Put enough salt for the mashed potatoes and add water. Mix well, cook for 2 to 3 minutes. You can put the cumin leaves and mix well. Now Masala is ready.
For making Dosa :
Grind the Rice and Daal together very smoothly.(Water should be added while grinding). Add salt to the liquid dough. Leave it for fermenting for about 5 to 6 hours. If the dough is too thick, then add small quantity of water to make it thin. Heat tawa and brush oil over it. When the Tawa is really hot, splash little water over it immediately pour the dough on it , spread it thin, with a circular motion. This will have to be very swift and will need a bit practice. After spreading the batter/dough , lower the heat and put oil around the edges so that it seeps under the dosa. When edges come up slightly put the desired masala filling in the centre and fold one edge over the other. Serve the tasty Masala Dosa with sambhar and chutney.
If you put Ghee instead of oil then Ghee Masala Dosa is more tastier .