When ready, preheat the oven to 400°F. Line a large baking sheet with parchment paper. Set aside.
Roll the pastry dough out onto a lightly floured surface into a large rectangle (15-inch x 20-inch), about ⅛-inch thick.
Cut the large rectangle into 12 equal-sized squares (about 5x5 inches each).
Add about 2 tablespoons of the apple mixture to the middle of each square.
In a small dish, whisk together the egg and water. Brush a little bit of the egg wash over the edges of the squares, saving some for the tops of the pastries.
Fold the pastry over from one corner to another, forming a triangle shape, using a fork to press and seal the edges.
Brush the tops of each pastry with egg wash.
Use a fork to press the seal the edges of the pastry.
Place the pastries onto the prepared baking sheet. Bake for 20-22 minutes, or until golden brown.
Allow the pastries to cool before adding the glaze.