The key to a perfect Masala Dosa is a properly fermented batter. Fermentation gives it a tangy flavor and a light, airy texture that makes the dosa crispy when cooked. While the preparation takes time, the result is well worth the effort.
Thoroughly wash the rice, lentils, and fenugreek seeds and soak them in separate bowls for 8-12 hours.
Drain and grind them into a smooth paste with a little water. Mix both batters together and add salt.
Pour the batter into a large bowl and let it ferment in a warm place for 8-12 hours until it doubles in volume and becomes bubbly and airy.
Boil the potatoes and mash them coarsely.
In a pan, heat the oil or ghee. Add mustard seeds and let them splutter. Then add curry leaves and chopped onion and sauté until the onion is translucent.
Add ginger-garlic paste and turmeric powder. Sauté briefly. Add the mashed potatoes, mix well, and season with salt.
Heat a non-stick tawa (griddle) over medium-high heat and lightly grease it with oil. If the pan is too hot, you can cool it down by sprinkling some water, which will sizzle and evaporate immediately.
Pour a ladle of batter into the center of the pan and quickly spread it outwards in a circular motion to form a thin crepe. Drizzle a little oil around the edges. Cook until the edges start to turn golden brown and lift off the pan.
Once the dosa is beautifully browned and crispy, place a generous spoonful of the potato filling on one half. Fold the dosa in half or roll it up and serve immediately.