INGREDIENTS

FOR THE DOSA BATTER:

• 2 cups short-grain rice
• ½ cup urad dal (split husked black lentils)
• 1 teaspoon fenugreek seeds
• ½ teaspoon salt
•  Vegetable oil, for frying

FOR THE POTATO FILLING:

• 3 tablespoons ghee or vegetable oil
• 1 teaspoon mustard seeds
• ½ teaspoon cumin seeds
• 2 small dried hot red peppers
• 1 medium onion, diced
• ½ teaspoon salt
• ½ teaspoon turmeric
•  Pinch of asafetida
• 1 tablespoon grated ginger
• 6 to 8 curry leaves
• 4 garlic cloves, minced
• 2 small green chiles, finely chopped
• 1 ½ pounds yellow-fleshed potatoes, such as Yukon Gold, boiled, peeled and cubed
• ½ cup roughly chopped cilantro, leaves and tender stems

PREPARATION

1. Make the dosa batter: Put rice in a bowl, rinse well and cover with 4 cups cold water. Put urad dal and fenugreek seeds in a small bowl, rinse well and add cold water to cover. Leave both to soak for 4 to 6 hours.
2. Drain rice and dal-fenugreek mixture in separate colanders. Put rice in a food processor, blender or wet-dry grinder. Add 1 cup cold water and grind to a smooth paste. It will take about 10 minutes, and it may be necessary to work in batches. Repeat the process with the dal-fenugreek mixture.
3. Combine the two pastes in a medium mixing bowl. Whisk together, adding enough water to obtain a medium-thick batter. You should have about 6 cups. Cover bowl with a kitchen towel and set in a warm place. Let ferment until the surface is bubbly, about 8 hours. Stir in the salt. Use the batter straight away or refrigerate for later use. (Batter will keep for up to a week, refrigerated. Thin with water if necessary before proceeding.)
4. Make the potato filling: Put ghee in a wide skillet over medium heat. When oil is wavy, add mustard seeds and cumin seeds. Wait for seeds to pop, about 1 minute, then add red peppers and onion. Cook, stirring until onions have softened, about 5 minutes. Season lightly with salt. Add turmeric, asafetida, ginger, curry leaves, garlic and green chile. Stir to coat and let sizzle for 1 minute.
5. Add potatoes and 1/2 cup water. Cook, stirring well to combine, until liquid has evaporated, about 5 minutes. Mash potatoes a bit with the back of a wooden spoon. Season well with salt, add cilantro, then set aside at room temperature. (Potato filling may be prepared up to a day in advance.)
6. To make dosas, set a griddle or cast-iron skillet over medium heat. Brush with about 1 teaspoon vegetable oil. Ladle 1/4 cup batter in the center of griddle. Using bottom of ladle, quickly spread batter outward in a circular motion to a diameter of about 7 inches. Drizzle 1/2 teaspoon oil over the top. Leave dosa batter to brown gradually until outer edges begin to look dry, about 2 minutes, cooking on one side only. With a spatula, carefully loosen dosa from griddle. Bottom should be crisp and beautifully browned. Spoon 1/2 cup potato filling onto top of dosa, centering it as a strip in the middle of the round dosa. Flatten the potato mixture slightly. Using the spatula, fold the sides of the dosa around the filling to make a cylindrical shape. Serve immediately. Continue making dosas one at a time.