Wash rice and urad dal separately and soak them in water for about 6-8 hours.
Grind them separately into a fine paste using water. Mix both pastes together, add salt, and let it ferment overnight or for about 8-10 hours in a warm place.
The batter should be of pouring consistency, not too thick nor too runny.
Heat oil in a pan. Add mustard seeds and let them splutter.
Add cumin seeds, asafoetida, and curry leaves. Sauté for a few seconds.
Add chopped onions, green chilies, and ginger. Sauté until onions turn translucent.
Add turmeric powder and mix well.
Add mashed potatoes and salt. Mix everything thoroughly. Cook for 2-3 minutes. Keep aside.
Heat a non-stick skillet or a dosa pan. Spread a ladleful of dosa batter in a circular motion to form a thin layer.
Drizzle some oil around the edges. Let it cook until the bottom turns golden brown and crisp.
Spread a spoonful of prepared potato filling in the center of the dosa.
Fold the dosa from both sides to cover the filling, forming a roll or a semi-circle.
Cook for another minute and then transfer to a serving plate.
Repeat the process with the remaining batter and filling.