Prepare the dosa batter by soaking 1 cup urad dal and 3 cups rice separately for 4-6 hours. Grind them separately into a smooth paste, mix them together, and ferment the batter overnight.
For the masala filling, boil 500 grams of potatoes, peel and mash them. Set aside.
Heat 2 tablespoons of oil in a pan, add 1 teaspoon mustard seeds, 1 teaspoon urad dal, and a pinch of asafoetida. Once they splutter, add 1 chopped onion, 2 chopped green chilies, and a few curry leaves. Sauté until onions turn translucent.
Add 1/4 teaspoon turmeric powder, mashed potatoes, and salt to taste. Mix well and cook for 2-3 minutes. Set aside.
Heat a non-stick tawa or dosa pan on medium flame. Pour a ladleful of dosa batter and spread it in a circular motion to make a thin dosa.
Drizzle a few drops of oil around the edges and cook until the dosa turns golden and crispy.
Place a portion of the masala filling in the center of the dosa and fold it into a roll or a triangle.
Serve hot with coconut chutney and sambar.