Blend soaked rice and urad dal with water to a smooth batter. Add salt and mix well. Let it ferment for 8-12 hours.

Heat oil in a pan, add mustard seeds and cumin seeds. Once they splutter, add onions and green chilies. Sauté until onions are translucent.

Add turmeric powder and mashed potatoes. Mix well and cook for a few minutes. Stir in coriander leaves and remove from heat.

Heat a non-stick skillet and pour a ladle of dosa batter. Spread it into a thin circle. Drizzle oil around the edges.

Cook until the edges turn golden brown, then place a portion of potato filling in the center. Fold the dosa over the filling and serve hot.