Soak 1 cup rice, 1/4 cup urad dal, and 1/2 tsp fenugreek seeds in water for 4 to 6 hours
Drain and grind to a smooth batter with enough water
Add salt and ferment the batter overnight or until doubled
Boil 3 to 4 potatoes until soft, peel, and mash them
Heat oil in a pan
Add mustard seeds, cumin seeds, curry leaves, green chilies, and chopped onions
Sauté until onions turn soft
Add turmeric powder and salt
Add mashed potatoes and mix well
Add chopped coriander leaves and cook for 2 minutes
Heat a dosa tawa or flat pan
Pour a ladle of batter and spread it thinly in a circle
Drizzle oil around the edges
Cook until the dosa turns golden and crisp
Place potato masala in the center
Fold the dosa over the filling
Serve hot with coconut chutney and sambar