Place the potatoes and salt in a large saucepan and add enough water to cover by 1 inch. Bring to a boil, then reduce the heat and simmer for 12 to 15 minutes until a paring knife can be inserted with little resistance. Drain and set aside to cool slightly.

Heat the oil in a 12-inch nonstick skillet over medium-high heat until shimmering. Stir in the fennel seeds, cumin seeds, mustard seeds, fenugreek, turmeric, and pepper flakes; cook until fragrant, about 10 seconds. Add the onion and salt and cook until the onion is softened, about 5 to 7 minutes.

Stir in the garlic and ginger and cook until fragrant. Add the cooled potatoes and cook for 5 to 7 minutes until the edges begin to brown, then fold in the peas.

Transfer the mixture to a bowl and cool to room temperature, then refrigerate for at least 1 hour until completely cold. Once chilled, stir in the cilantro and lemon juice, seasoning with additional salt and pepper to taste.

Pulse the flour and salt in a food processor to combine. Drizzle the yogurt and oil over the flour and process for about 5 seconds until the mixture looks like coarse cornmeal. With the processor running, slowly add the water until a soft, malleable dough ball forms.

Transfer the dough to a floured counter and knead by hand for 2 minutes until slightly firm. Wrap in plastic wrap and let rest for at least 20 minutes to allow the gluten to relax.

Divide the dough into 12 equal pieces. Working one at a time while keeping the others covered with greased plastic, roll a piece into a 5-inch round and cut it in half to form two half-moons.

Moisten the straight side of a half-moon with a wet finger, fold it in half, and press to seal the straight seam only, creating a cone. Fill the cone tightly with 2 tablespoons of filling, leaving a small rim at the top. Moisten the inside rim, pinch the edges together to seal, and crimp with a fork.

Heat the frying oil in a large Dutch oven to 375°F. Fry 8 samosas at a time for 2 1/2 to 3 minutes until golden brown and bubbly, adjusting the heat to maintain the oil temperature. Transfer to a paper towel-lined baking sheet and keep warm in a 200°F oven while finishing the remaining batches.

Serve hot, ideally with mint or tamarind chutney.