Soak rice, urad dal, and a little fenugreek seeds separately for 4 to 6 hours
Grind urad dal and fenugreek to a smooth batter
Grind rice to a slightly coarse batter
Mix both batters with salt and enough water to make a pourable consistency
Ferment the batter overnight or until doubled
Boil potatoes until soft
Heat oil in a pan
Add mustard seeds, cumin seeds, curry leaves, green chilies, and chopped onions
Add turmeric powder and salt
Add boiled, mashed potatoes and mix well
Add chopped coriander leaves and set the filling aside
Heat a dosa tawa or flat pan
Pour a ladle of batter and spread it thin in a circle
Drizzle oil or ghee around the edges
Cook until the dosa turns golden and crisp
Place potato filling in the center
Fold the dosa over the filling
Serve hot with coconut chutney and sambar