Prepare the potato sabzi: Fill a medium pot about two-thirds of the way with water. Bring to a boil and add the potato; let cook until potato feels just tender when a paring knife is inserted into the center, about 25 minutes. Remove and let cool slightly, then peel and break into bite-size pieces.
In a pan set over medium heat, add the 2 tablespoons ghee or oil. Once hot, add the mustard seeds and cook, giving the pan a good shake every few seconds, until the seeds pop and sputter, about 2 minutes. Lower the heat to medium-low and add the chana dal and urad dal. Cook, stirring occasionally, until the dal is fragrant and lightly golden, 2–3 minutes. Add the onions and chiles and cook, stirring occasionally, until the onions are tender and translucent, 6–8 minutes. Add the potatoes, ¾ teaspoon kosher salt, and the turmeric and stir well; cook until the potatoes have absorbed the spices and turned a pale yellow, 8–10 minutes. (If the potatoes are looking pasty and dry, add water, mashing in 1 tablespoon at a time until they achieve a lumpy but pliable consistency.) Taste and adjust the salt as desired.
Remove the filling from the heat and stir in the cilantro. Add the lemon juice to taste. Transfer to a bowl, cover, and set aside or refrigerate until you are ready to cook the dosas.
Meanwhile, in a small bowl, mix the gunpowder paste: Add the chutney powder and ghee or oil. Mix until a smooth paste forms, then set aside. (Gunpowder paste will keep for up to a few weeks in the refrigerator.)
Heat a 12-inch nonstick pan or well-seasoned cast-iron griddle over medium-high heat, or set a nonstick electric griddle to medium-high. (If using cast iron, grease the surface lightly with oil.) Sprinkle a few drops of water on the hot surface: It is ready when the drops of water sizzle and evaporate right away. Be sure the dosa batter is well-stirred and free of lumps, then ladle ⅓ cup of batter into the center of the griddle. Working quickly but evenly, and using the bottom of the ladle, form the dosa by making a continuous spiral motion, starting in the center and dragging the batter outward toward the pan’s edges to form a large, connected bull’s-eye shape 10 inches in diameter. (If desired, you can trickle and drag more batter around the edges to fill the pan, and patch any too-thin areas or small holes with additional batter.) When the top develops bubbles and looks just set, quickly brush or drizzle a little ghee or oil (about 1 teaspoon) over the top as the dosa cooks, taking care to saturate the outer areas. When the bottom is lightly crispy and golden brown, about 5 minutes, you can fill the dosa, or use a spatula to flip the dosa and crisp the remaining side if desired. (This is helpful if you’re new to dosa making and want to be sure thicker spots are properly cooked through.) If flipping, drizzle an additional teaspoon of oil around the edges after flipping, and cook the other side until the thicker ridges of batter are set but still flexible, 30–60 seconds. Flip the dosa again so that the crispier, browner side is on the bottom. Spread 1 tablespoon of the gunpowder filling over the lighter surface of the dosa.
Fill the dosa on the pan if desired: Spoon ¼ cup of the potato sabzi onto the center of the dosa on its lighter side, using the back of the spoon to spread and press the filling into a thin layer so that it heats evenly. Carefully loosen the perimeter of the dosa with a flat spatula, then slide the spatula beneath the center to dislodge. Use the spatula to coax one edge of the dosa gently over to the other side, tucking the edge slightly on the inside of the crêpe to create a tall cylindrical roll. Alternatively, you can fold the dosa into thirds like a letter.
Remove to a plate and serve immediately with coconut chutney and additional gunpower paste and potato sabzi if desired. Eat with your hands.