Heat the oil in a large saucepan over medium heat. Add the onion and cook, stirring occasionally, until soft and golden brown, about 8 minutes. Add 3/4 teaspoon salt, 1/2 teaspoon pepper, and the curry powder and cook, stirring, for 1 minute. Remove from heat and stir in the potatoes and peas.
Unroll the piecrusts and cut each into 6 triangles. Place a heaping tablespoon of the potato mixture in the center of each piece. Gather the corners of the dough and pinch to form a point. Pinch the seams to seal. Transfer to a baking sheet.
Bake the samosas until golden, about 25 minutes. Serve with the mango chutney, if desired.