To start, defrost the puff pastry overnight in the refrigerator, or at room temperature for about 30 minutes. Preheat oven to 375ºF.
Cover the potato with 1" of water in a medium saucepan. Cook until tender, then drain in a colander and coarsely mash it with a fork. Set aside.
Return saucepan to medium heat and melt the butter. Add onion and cook about 6 minutes, until translucent and just starting to brown.
Add jalapeño and ginger, cook a minute more, then stir in the spices and salt. After about 30 seconds (when spices smell aromatic) add mashed potato, frozen peas and cilantro; stir until well combined, then transfer to the fridge to cool.
While the filling cools, line two large baking sheets with parchment paper. Grab the first piece of dough from the box and roll it into a square of about 13x13" on a lightly floured surface.
Take a bowl that's about 6" diameter and flip it over to make a circle template. Using a sharp knife, trace around the outside of the overturned bowl to make your 4 pastry circles. Transfer them to the prepared baking sheet, and place in the freezer while you repeat this process with the second square of puff pastry dough. Once you've made four more circles, transfer them to the second baking sheet, put that one in the freezer, and remove the first sheet.
Prick pastry several times with a fork, then cut the circles in half (a bench scraper does the trick nicely, but so will a sharp knife). Spoon about 1 tablespoon of filling onto bottom half of each half-circle.
Brush very lightly with egg wash all around the edges, then fold over and press down firmly to seal. Chill the samosas in the freezer while you repeat the filling process with the second batch. (And if at any point the dough is becoming too soft while you're working, simply pop the whole baking sheet back in the freezer for a few minutes.)
When all the samosas are filled, brush them all over with egg wash, and arrange on a single large baking sheet. Bake for 15-20 minutes, until puffed and golden brown. Let cool about 5 minutes before serving (with chutney for dipping!).