Soak the urad dal in water for at least 4 hours, then drain and blend with rice flour and a splash of water until a smooth, thick batter forms.
Transfer the batter to a bowl, cover, and let it ferment in a warm spot for 8 to 12 hours until it becomes slightly airy and tangy.
Mash the boiled potato into coarse chunks and set aside.
Sauté the finely diced onion, grated ginger, and sliced scallions in half of the avocado oil until fragrant and translucent.
Stir the mashed potatoes into the aromatics, seasoning with a pinch of salt or turmeric if desired, then remove from heat.
Heat a large non-stick skillet over medium heat and lightly coat with the remaining avocado oil.
Pour a ladle of the fermented batter into the center of the pan and use the back of the spoon to spread it into a very thin, even circle.
Cook for 2-3 minutes until the bottom is deeply golden and the edges begin to curl away from the pan.
Place the potato mixture in the center of the crepe, fold it in half or roll it up, and serve immediately while the exterior is perfectly crisp.