Make the Filling
Wash the leeks thoroughly to remove any grit, trim the ends, and slice them into very thin rounds, about 1/8–¼ inch thick.
Heat the oil and butter in a large frying pan or skillet over medium-high heat.
Once hot, add the black mustard seeds and wait for them to pop.
Add the finely sliced onions and fry for about 10 minutes, stirring occasionally, until they are soft, golden, and sweet.
Stir in the grated ginger and cook for one minute until fragrant, then add the sliced leeks to the pan.
Cook for another 10 minutes, stirring, until the leeks are completely soft and have reduced in volume.
Add the peas, garam masala, cumin, ground red chile, turmeric, salt, and lemon juice.
Mix everything thoroughly and cook for 2 more minutes.
Remove the pan from the heat and allow the filling to cool. Just before you're ready to assemble the samosas, stir in the freshly chopped mint.
Assemble & Bake the Samosas
Preheat your oven to 400°F / 200°C and line two large baking sheets with parchment paper.
Carefully unroll one sheet of filo dough onto a large cutting board.
Keep the remaining sheets covered with a damp kitchen towel to prevent them from drying out. Lightly brush the sheet with melted butter and place another sheet of filo directly on top.
Using a sharp knife, cut the layered filo stack lengthwise into three long, equal strips (each should be about 4 x 10 inches).
Working with one strip at a time, take one corner and fold it over to create a small cone shape at the end of the strip. Spoon 1 heaped tablespoon of the leek filling into the cone.
Fold the open side of the cone over the filling to seal it, forming a triangle.
Continue folding the triangle over on itself, flag-style, until you reach the end of the strip.
Trim any excess dough and seal the final flap with a little melted butter.
Place the finished samosa on your prepared baking sheet and repeat with the remaining filling and filo.
Once all samosas are folded, brush the tops and sides with the remaining melted butter and sprinkle with nigella or cumin seeds, if using.
Bake for 15 minutes, or until the samosas are golden brown and perfectly crispy.
Serve immediately with your favorite chutney or raita.
Keep filo dough covered with a damp cloth while you work to prevent it from drying out and cracking.
Don't overfill the samosas. About one heaping tablespoon is the perfect amount to ensure they seal properly and don't burst while baking.
