Prepare the dosa batter by soaking 2 cups of rice, 1/2 cup of urad dal, and 1/4 cup of chana dal in water for 4-5 hours. Drain the water and grind the mixture into a smooth batter. Add 1/2 teaspoon of salt and let it ferment overnight.
For the potato masala, boil 4-5 medium-sized potatoes, peel and mash them. Heat 2 tablespoons of oil in a pan, add 1 teaspoon of mustard seeds, 1/2 teaspoon of urad dal, and a pinch of asafoetida. Once the seeds splutter, add 1 chopped onion, 2 chopped green chilies, and a few curry leaves. Sauté until the onions turn translucent.
Add the mashed potatoes, 1/4 teaspoon of turmeric powder, salt to taste, and 1/4 cup of water. Mix well and cook for 5-7 minutes. Set aside.
For the red chutney, blend 1 cup of grated coconut, 1/4 cup of roasted chana dal, 5-6 dried red chilies, 1/2 teaspoon of tamarind paste, and salt to taste with some water to form a smooth paste.
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 form a thin dosa. Drizzle some oil around the edges and cook until the dosa turns golden brown.
Spread 1-2 tablespoons of red chutney over the dosa and place a portion of the potato masala in the center. Fold the dosa and serve hot with coconut chutney and sambar.