In a large bowl, mix together the flour, salt and oil. Add water, 1 tablespoon at a time, until mixture becomes crumbly.
On a floured surface, knead to a soft pliable dough, about 5 minutes. Cover with a damp towel and allow to rest for 20 minutes.
In a saute pan set to medium heat, add 2 teaspoons oil and cumin seeds. Cook until they crack and become fragrant. Add onions and saute until cooked and transparent, about 5 minutes. Add turmeric, ginger, garam masala, green chiles, cilantro, lemon juice, potatoes, peas and a pinch of salt. Stir-fry for 2 minutes, adding the additional 1 teaspoon oil if necessary, until potatoes have softened. Set aside and allow to cool.
Divide dough into 10 equal portions. Roll a piece of dough into a 5-inch oval, 1/4 inch thick. Cut into 2 halves, and roll around your finger to make a cone. Place a tablespoon of the filling into the cone, seal the third side using a moist finger.
In a heavy-bottomed skillet over medium-high heat, pan-fry samosas for about 4 minutes on each side, until golden brown. Work in small batches of 5 to 6, allowing each samosa to have room in the pan.