For the Turnovers:

1. Melt the butter in a large skillet over medium heat. Add the apples and sauté for a few minutes, stirring often. Incorporate the brown sugar, cinnamon, and salt, ensuring the apple pieces are well-coated. Continue cooking for 5 to 10 minutes until the apples soften and become syrupy. Lower the heat.
2. Sprinkle in the cornstarch and mix until the juices come to a boil and thicken, which should take about 1 minute. Transfer the mixture to a bowl and allow it to cool for 15 minutes, then refrigerate until chilled, roughly 30 minutes.
3. Once the apples are chilled, preheat your oven to 400°F. Prepare two baking sheets by lining them with parchment paper.
4. On a lightly floured surface, roll each thawed puff pastry sheet into a square measuring 12 inches. Cut each sheet into 4 equal squares.
5. Position a puff pastry square on the baking sheet and brush the edges with the egg wash. Spoon a few tablespoons of the apple filling onto each square. (You may have some filling left over depending on the size of your apple pieces and how much they cooked down.) Fold one corner of the pastry over the filling to meet the opposite corner, forming a triangle. Seal the edges by crimping with a fork along both straight sides. Brush the top with additional egg wash and cut a few small slits in it. Optionally, sprinkle with sugar.
6. Bake for 18 to 20 minutes, or until the pastry is golden and puffed. Allow to cool for at least 15 minutes before serving.

For the Glaze:

1. In a small mixing bowl, whisk together the powdered sugar and water until smooth. If necessary, add a little more water to achieve the desired consistency. Drizzle the glaze over the warm turnovers and enjoy!