Place the potatoes in a large saucepan with a pinch of salt and cover with water by 1 inch. Bring to a boil over high heat, then reduce to a simmer and cook until tender but still holding their shape, about 10 to 12 minutes.

Drain the potatoes well and set them aside while you prepare the spice base.

Heat the vegetable oil in a large skillet over medium-high heat until shimmering. Add the fennel seeds, cumin seeds, mustard seeds, fenugreek, turmeric, and Kashmiri chili powder and cook until the mustard seeds begin to pop.

Stir in the onion and cook until softened and translucent, about 5 minutes. Add the garlic and ginger and cook until fragrant.

Add the potatoes to the skillet and cook, mashing slightly with a spatula, until the edges are browned and crisp, 5 to 8 minutes. Stir in the peas, cilantro, and lemon juice, then season with salt and pepper to taste. Transfer the filling to a bowl to cool completely.

Whisk the flour and salt together in a large bowl. In a separate bowl, whisk the yogurt and vegetable oil together, then stir into the flour mixture. Gradually add the cold water and knead until the dough forms a smooth, elastic ball. Cover with plastic wrap and rest for at least 30 minutes.

Divide the dough into 12 equal pieces and roll each into a ball. On a lightly floured surface, roll each ball into a 6-inch circle, then cut each circle in half to create 24 semi-circles.

Brush the straight edge of one semi-circle with water. Fold it in half to form a cone, overlapping the edges and pressing to seal. Fill with the potato mixture, brush the top open edge with water, and pinch firmly to seal the turnover.

Heat 3 quarts of vegetable oil in a large Dutch oven to 350°F. Fry the samosas in batches of 6 until deep golden brown and crispy, about 3 to 5 minutes per batch.

Transfer the finished samosas to a paper towel-lined plate using a wire spider or slotted spoon. Serve hot, while the pastry is still crackling.