Sift the flour, baking powder and salt into a large bowl and make a well in the centre.
Rub the oil into the flour, until the mixture resembles coarse breadcrumbs. Add the warm water, a little at a time and knead into the flour adding more water as required to make a soft, pliable dough. Knead dough for 4 - 5 minutes, until smooth.
Cover the dough with plastic wrap or cling film and leave to rest for at least half hour, or overnight in the fridge. Bring back the dough to room temperature before making samosa shells.
Heat the oil in a saute pan, then add the cumin seeds. When seeds sizzle, add the onion, and fry for 5 - 7 minutes, until the onion is soft but not coloured.
Add the ginger, turmeric, ground cumin, amchur and garam masala. Fry together for a couple minutes, then add the potatoes.
Saute the potatoes in the onion-spice mixture for about 5 - 7 minutes, until they begin to fray around the edges. Add enough water to cover the potatoes, then simmer for about 15 - 25 minutes, until the potatoes are very tender.
Add the peas, and cook for a few minutes. Season to taste with the salt and lemon juice.
Take off the heat, and mash coarsley. Stir in the cilantro. Let the filling cool completely before stuffing into samosas.