Boil the diced potatoes in a pot of salted water until tender, about 10 minutes. Drain and set aside.
Heat the oil in a pan over medium heat. Add the cumin seeds and mustard seeds (if using). Cook until they start to crackle.
Stir in the ground coriander, ground cumin, turmeric, red chili powder, and salt. Cook for 1 minute.
Add the boiled potatoes and peas. Cook for 5 minutes, mashing the potatoes slightly.
Stir in the garam masala, chili flakes (if using), and lemon juice. Adjust seasoning to taste. Garnish with fresh coriander leaves and set aside to cool.
In a large bowl, mix together the flour, salt, and vegetable oil or ghee. Rub the mixture between your fingers until it resembles coarse crumbs.
Gradually add water, a little at a time, until a smooth, pliable dough forms. Knead for a few minutes, then cover with a damp cloth and let it rest for 20 minutes.
Divide the dough into small balls and roll each ball into a thin circle (about 6 inches in diameter) on a lightly floured surface.
Cut the circle in half to form two semi-circles. Fold each semi-circle into a cone shape, sealing the edge with a little water.
Fill the cone with the prepared filling and seal the open edge by pinching the sides together. Ensure the filling is well-sealed inside the dough.
Heat vegetable oil in a deep pan or fryer over medium heat.
Fry the samosas in batches, turning occasionally, until they are golden brown and crispy, about 4-5 minutes.
Remove with a slotted spoon and drain on paper towels.
Enjoy your meal!