Rinse the rice until the water runs clear and soak it 6 hours. Separately soak the urad dal and fenugreek 4 hours.
Grind the dal and fenugreek first with just enough cold water to make a light, fluffy batter — it should almost float. Grind the rice separately to a fine, slightly grainy paste. Fold the two together by hand with the salt.
Cover loosely and ferment at warm room temperature 10-14 hours, until the batter has risen, smells pleasantly sour, and is full of bubbles. Refrigerate until you are ready; thin with water to a pourable single-cream consistency before cooking.
For the masala, heat the ghee in a wide pan over medium. Add the mustard seeds and wait for them to pop, then add the LXV Panch Phoron Spice Blend and the curry leaves — stand back, they will spit.
Add the onions, chilies and ginger and cook 8 minutes until soft and just golden. Stir in the turmeric.
Fold in the broken potatoes and salt, adding 1/4 cup water to loosen. Cook 5 minutes, pressing some of the potato into the pan so it picks up color. Finish with lime and cilantro and keep warm.
Heat a cast-iron or carbon-steel pan over medium-high until a drop of water skitters. Wipe with a lightly oiled cloth. Pour a ladle of batter in the center and spread it outward in a quick spiral to a thin round.
Drizzle a teaspoon of ghee around the edge and cook 2-3 minutes without touching it, until the underside is lace-brown and the edges lift. Do not flip a dosa you plan to fill.
Spoon masala down the center, fold, and serve immediately — a dosa waits for no one. Repeat, wiping the pan between each.