Heat the oil in a large skillet over medium-high heat. Add the onion, garlic, and potato and cook, stirring occasionally, until softened, about 8 minutes.
Add the chicken and continue to cook, stirring frequently and breaking up the meat with a spatula, until the meat is browned, about 5 minutes more. Stir in the curry powder, fish sauce or soy sauce, salt, sugar, and cayenne. Stir in the water and peas and cook, stirring, until the peas are heated through and the liquid is mostly evaporated.
Preheat the oven to 400ºF (if using an air fryer, there’s no need to preheat).
Roll the pastry out to about 1/8- to ¼-inch thick and cut out circles using a 3 ½-inch round pastry cutter (or do what I do and use the ring from a wide-mouth canning jar). You should have 12 rounds.
Spoon about 2 tablespoons of the filling mixture onto each of the pastry rounds, fold them over, and crimp the edges with a fork. Brush with the egg wash.
Bake in the preheated oven for 15 to 20 minutes, until the crusts are golden brown. Remove from the oven and let cool on a wire rack. Serve warm or at room temperature.