Prepare the Dosa Batter: Rinse 300g (1.5 cups) of Idli rice thoroughly under cold running water until the water runs clear. Place the rinsed rice in a large bowl and cover with ample water. Let it soak for at least 4-6 hours, or preferably overnight. In a separate bowl, rinse 75g (1/3 cup) of Urad Dal and 1 tsp of fenugreek seeds together until the water is clear. Cover with water and soak for at least 4-6 hours, or overnight.

Grind the Batter: Drain the soaked Urad Dal and fenugreek seeds, reserving the soaking water. Transfer the drained dal and fenugreek seeds to a wet grinder or a high-powered blender. Add a little fresh water and grind to a very smooth, fluffy paste. This may take 10-15 minutes in a wet grinder or 5-8 minutes in a blender, adding water sparingly as needed. Scrape down the sides periodically. In a separate grinder jar, drain the soaked rice and grind it into a slightly coarse paste. Do not make it too smooth; a little texture is desired. Add water gradually as needed.

Combine and Ferment: Transfer the ground rice paste and the ground dal-fenugreek paste into a large, clean bowl (preferably non-metallic). Add salt to taste. Mix everything thoroughly with your hands or a spatula until well combined. The mixture should be thick but pourable. Cover the bowl with a lid, leaving a small gap for air, or use a breathable cover like cheesecloth. Place the bowl in a warm place (ideally 25-30°C or 77-86°F) to ferment for 8-12 hours. The batter will rise and develop a slightly sour aroma.

Prepare the Potato Masala: While the batter ferments or just before making the dosas, prepare the potato filling. Boil 500g (1 lb) of potatoes until fork-tender. Drain them, let them cool slightly, peel, and then roughly mash them with a fork or potato masher. Do not mash them into a fine paste; some chunks are desirable. Heat 2 tbsp (30ml) of vegetable oil or ghee in a pan over medium heat. Once the oil is hot, add 1 tsp of mustard seeds. Let them splutter. Add 1 tsp of cumin seeds and let them sizzle for a few seconds. Add 1/4 tsp of asafoetida (hing) and 10-12 curry leaves. Sauté for about 30 seconds until fragrant.

Sauté Aromatics and Spices: Add 2 finely chopped medium onions to the pan. Sauté until they become translucent, about 5-7 minutes. Add 2-3 finely chopped green chilies and 2.5cm (1 inch) piece of grated or finely chopped ginger. Sauté for another 1-2 minutes until the raw smell of ginger disappears. Add 1/2 tsp of turmeric powder and salt to taste. Mix well and cook for 30 seconds.

Combine Filling: Add the roughly mashed potatoes to the pan. Mix everything gently but thoroughly, ensuring the spices coat the potatoes evenly. Add about 1/4 cup (60ml) of water to achieve a moist but not watery consistency. Cook for another 3-5 minutes, stirring occasionally, until the filling is well combined and heated through. Garnish with 2 tbsp of chopped fresh cilantro. Remove from heat and set aside.

Prepare for Dosa Making: Once the batter is fermented, gently stir it. If it's too thick, add a little water (a tablespoon at a time) to reach a pouring consistency, similar to thin pancake batter. The batter should spread easily but not be watery. Heat a non-stick dosa griddle or a cast-iron tawa over medium-high heat. It's crucial to get the griddle temperature right. To test, sprinkle a few drops of water; they should sizzle and evaporate quickly.

Cook the Dosa: Lightly grease the hot griddle with oil or ghee. If using a cast iron tawa, you might need to wipe it with a damp cloth between dosas to control the heat. Pour a ladleful (about 1/4 cup or 60ml) of batter onto the center of the hot griddle. Immediately start spreading the batter in a thin, circular motion, outwards from the center, to form a large, thin crepe. Aim for a diameter of about 20-25cm (8-10 inches).

Cook and Fill: Drizzle about 1-2 tsp (5-10ml) of oil or ghee around the edges and on top of the dosa. Cook for about 1-2 minutes, or until the bottom turns golden brown and crispy, and the edges start to lift. You will see small bubbles forming on the surface. Once the bottom is cooked and the top looks cooked through (no longer liquid batter), spoon about 2-3 tbsp of the prepared potato masala onto one half of the dosa. Carefully fold the other half of the dosa over the filling, or roll it up.

Serve: Gently slide the finished masala dosa onto a serving plate. Serve immediately while hot and crispy. Repeat the process with the remaining batter and potato masala, greasing the pan lightly between each dosa. Serve with sambar and coconut chutney for a complete South Indian meal.