Rinse rice and soak in water for 4-6 hours.

Rinse urad dal and fenugreek seeds and soak in water for 4-6 hours.

Drain rice and dal-fenugreek mixture.

Grind rice with water to a smooth paste.

Grind dal-fenugreek mixture to a smooth paste.

Combine the two pastes in a mixing bowl and whisk together, adding enough water to obtain a medium-thick batter.

Cover and let ferment for about 8 hours in a warm place.

Stir in salt.

Refrigerate for later use, or use immediately.

To make the potato filling, heat ghee or vegetable oil in a skillet over medium heat.

Add mustard seeds and cumin seeds, and wait for them to pop.

Add red peppers and onion, and cook until softened.

Season with salt.

Add turmeric, asafetida, ginger, curry leaves, garlic, and green chile and stir to coat for 1 minute.

Add potatoes and 1/2 cup water.

Cook, stirring well to combine, until liquid has evaporated.

Mash potatoes with a wooden spoon.

Season with salt and add cilantro.

Set aside at room temperature.

To make dosas, set a griddle or cast-iron skillet over medium heat.

Brush with vegetable oil.

Ladle batter in the center of the griddle and spread in a circular motion to a diameter of about 7 inches.

Drizzle 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, loosen dosa from the griddle. The bottom should be crisp and browned.

Spoon potato filling onto top of dosa in a strip in the middle.

Flatten the potato mixture slightly.

Fold the sides of the dosa around the filling to make a cylindrical shape.

Serve immediately and continue making dosas one at a time.