The first thing you have to do is Soak rice and urad dal in separate bowls for 4-6 hours and strain it.
Process rice and urad dal mixture in commercial grinder or food processor. While grinding, add water a cup at a time until it reaches the consistency of pancake batter.
Salt to taste.
Cover these mixture and let sit overnight. Don't refrigerate. it needs to ferment.
Add some oil or ghee to a skillet and after it heates, Add mustard seeds and chana dal.
Shake mixture over the flame until golden brown color appears.
Add curry leaves, green chiles, and onions.
Sprinkle a little bit turmeric powder and add salt for taste then Stir.
Break up boiled potato into small chunks and add them to your mixture. pour some water and stir it.
Pour refrigerated dosa mixture into a flat bottom bowl.
Pour batter into a greased skillet.
It form a circular shape with the batter using the flat bottomed dish.
Brush on ghee to frying dosa.
Place masala filling near the center of the dosa.
Begin rolling from the edge of the dosa as you would a wrap.
Remove dosa from hot top or skillet.
Masala Dosa is generally served with coconut, cilantro and tomato chutney as well as sambar (vegetable lentil stew)