Soak rice, urad dal, and fenugreek in water to cover for 3 hours. Rinse under running water and drain. Transfer to a food processor fitted with the steel blade, and grind to a smooth, yet thick batter. Set batter aside, and let ferment overnight at room temperature. Add salt and a little water if necessary to thin batter.

Make the Potato and Pea Filling: In a large saucepan, place potatoes, 1 teaspoon salt, and water to cover. Bring to a boil. Reduce heat, and simmer until fork tender. Drain, peel, and cut potatoes into 1/4-inch pieces.

Heat oil in a large skillet over medium heat. Add mustard seeds, chana dal, cashews, ginger, garlic, onions, and curry leaves. Saute until onions are soft, 3 to 4 minutes. Add curry powder, chile powder, turmeric, and corriander. Cook for 1 minute. Add peas and potatoes. Add 2 tablespoons water, and cook until heated through. Season with salt, and add chopped cilantro. Keep warm while making the dosa.

Heat griddle to 350 degrees. Sprinkle hot griddle with water and a teaspoon of oil. Wipe clean with a cloth. Drizzle a little oil on the griddle. Spread evenly over the surface of the griddle, removing any excess with a bench scraper.

Using a 1-cup measuring cup or ladle, measure 3/4 cup batter, and pour onto grill. Spread batter out into a 16-inch oval using the bottom of the measuring cup. Using a scraper, remove any excess batter from the top of the dosa. Cook until the surface bubbles and the bottom is golden brown, 1 to 1 1/2 minutes. Place filling crosswise in center of dosa. Fold dosa in thirds to enclose filling.

Serves 6.