Bring a large pot of salty water to a boil and cook potatoes and carrots for about 10 minutes, until tender. Drain water and mash vegetables with a potato masher or the back of a wooden spoon plus some elbow grease. The mixture can still be a little chunky.
While potatoes and carrots are boiling, heat the olive oil in a pan over medium heat. Add the onion, ginger, and ground cumin and cook for about 5 minutes. Everything about that will smell good.
Dump onion mixture into potato mixture. Add peas, cilantro, mint, garam masala, serrano, and salt. Mix together into a beautiful, chunky, fragrant mush. Allow to cool slightly. Try some with a spoon, no one will know.
Preheat oven to 375° F. Line a baking sheet with parchment paper.
On a lightly floured surface, unfurl a sheet of cold puff pastry and slice it into 3- by 3-inch squares. Roll each square out only slightly with a rolling pin, dollop a generous scoop of filling on one diagonal half, and then fold it into a triangle, one far corner folding up to meet its opposite. Crimp the edge with the tines of a fork to seal. If you run out of pastry but are still left with filling, unfurl the other sheet and continue 'till one resource is spent.
Line up triangles on your baking sheet (may need to work in shifts depending on how big you make your triangles and how big your sheet is!). Brush the tops with a little egg wash and then bake 30 to 40 minutes, until golden. Allow to cool slightly on the sheet, and then move to a wire rack for the remainder of the cooling process.