Preheat the oven to 400F. Line two baking sheets with parchment paper.
In a large skillet over medium heat, melt the butter. Add the apples, and cook for a few minutes while stirring frequently. Add the brown sugar, lemon juice, cinnamon, and salt and stir to coat apple pieces. Cook for about 5 minutes or until the apples are softened and become syrupy. Reduce the heat to low.
Sprinkle in the cornstarch and stir until the juice boils and thickens, about 1 minute. Transfer to a bowl. Let cool for 15 minutes then refrigerate until chilled, about 30 minutes.
When the apples are chilled, roll each thawed puff pastry sheet out on a lightly floured surface into a 12-inch square. Cut each sheet into 4 equal squares.
In a small bowl, beat the egg and water together, then brush the edges of each square with the egg wash.
Place a few tablespoons of filling in each square. Fold one point of the puff pastry over the filling to the opposite point, creating a triangle. Crimp and seal in the filling by pressing the lines of a fork along both straight edges. Brush the top with more egg wash and cut a few small slits in the top. Sprinkle with sugar, if desired.
Bake for 18 to 20 minutes or until the pastry is golden and puffed. Let cool for at least 15 minutes.
For the Glaze:
In a small mixing bowl, combine the powdered sugar and water and whisk until smooth. You can add a bit more water if the consistency needs to be thinned out. Drizzle over the warm turnovers and enjoy!
Notes
Try to dice the apples as uniformly as possible to ensure they cook evenly. Avoid chopping the apples larger than a ½ inch as they’ll be too large to keep the pastry from popping open in the oven.
Make sure to seal the turnovers tightly, so the apple filling does not spill out.
Don’t skip cutting a few slits on the puff pastry for ventilation so the steam can come out, and you can avoid the turnovers bursting in the oven.
If your thawed puff pastry is too difficult to work with, you can place it back into the fridge to help it firm up a bit.
Avoid overworking the puff pastry, as you’ll lose some flakiness when baked.
It might seem like a lot of apples at first in your skillet, but they will shrink as you continue to cook them.
Make sure you give the apple turnovers enough time to cool before adding the glaze. The glaze will melt off if the turnovers are piping hot.
You can add the glaze to a piping bag or ziptop bag with the corner cut to easily drizzle the glaze over the turnovers.
If you do not plan on baking the turnovers immediately, freeze them. If you assemble them ahead of time and leave them sitting, the turnovers will come out soggy.