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. Heat griddle to 350 degrees F. 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.
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 ground coriander. Cook for 1 minute. Add peas and potatoes. Add 2 tablespoons water, and cook until heated through. Season with salt, and add chopped coriander. Use as a filling for dosa. Yield: 6 servings
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