Preheat the oven to 375°F, and line two large baking sheets with parchment paper.

Stir, and cook the apples until they begin to simmer, then reduce the heat to low-medium. Continue to cook for about 10 minutes, stirring frequently. While the apples cook make the cornstarch slurry by mixing 2 tablespoons of cornstarch to 1 tablespoon of cold water.

Add the slurry to the cooked apples, and stir until thickened. Remove from heat, and allow the apple filling to cool for several minutes. Make the egg wash while the filling cools.

Whisk together the egg and 1 tablespoon of water. Set aside.

Remove the puff pastry from the refrigerator, and roll it out onto a flat workspace. Cut each sheet into 6 equal squares. Brush the egg wash around the edges of each square.

Spoon 2 to 3 tablespoons of filling into the center half of one of the squares (see photo #4 above). Fold the square in half diagonally to match up the edges. Use a fork to crimp and seal the edges, then apply egg wash to the tops of the turnovers. Sprinkle each with sanding sugar.

Transfer the turnovers to the baking sheets. Bake for 15 to 20 minutes or until lightly browned. Allow the turnovers to cool for 10 to 15 minutes before serving.