Firstly, wash, peel, and dice apples into about 1/3 inch cubes.
Then, in a medium pot melt butter and add diced apples to the pan.
Cook apples until softened. Make sure to stir occasionally so that nothing gets stuck or burnt to the bottom of the pan. This should take about 4-6 minutes.
Lower the heat to medium-low, and add in the brown sugar, cinnamon, and salt. Simmer the apple mixture until the apples are soft. This should take 2-4 minutes.
Then, remove apples from heat. Let cool to room temperature before assembling the turnovers. (Make your puff pastry during cooling.)
Line a baking sheet with parchment paper.
Make egg wash by beating together 1 egg and 1 TBSP water, mix, and set aside.
Once the puff pastry has chilled, take it to a worksurface. Dust work surface with some flour.
Roll out the completed puff pastry sheet into a 12 inch square. Using a pizza cutter, cut into thirds each way which should leave you with 9 4x4 inch squares. Using a pastry cutter make sure the bottoms of each square are not stuck to the work surface.
Make an imaginary line in your head diving each square in half diagonally from corner to corner. (Don't cut it, just know you won't put filling on one half.)
Next, place about 1 TBSP of apple filling mixture onto half of the pastry. Make sure to leave about 1/2 inch of clearance around the edges so you can "glue" them together with your egg wash.
Brush egg wash lightly around the edges of the half of the pastry that has the filling. Bring the pastry edges together folding on the imaginary line. Lightly pinch edge together being careful not to push the filling out. (Keep this egg wash, you will use it later.)
Then, using a fork, gently press all around the edges to seal (don't press the folded edge.) Do this with each turnover and place them on the lined baking sheet. Make sure they are about 1 inch apart to allow for the turnovers to rise and puff in the oven.
Cover the turnovers and refrigerate for 15 minutes to allow everything to chill and set. (this will help prevent the filling from leaking out during baking.)
Preheat oven to 425*F during this chilling time.
Once chilled. using a knife cut a few small slits over the filling to allow for steam to escape during baking.
Then, brush the tops with the egg wash, and *optional - sprinkle with sanding or Turbinado sugar.
Bake the Turnovers for 20-25 minutes, until golden brown and puffed.
Let cool on the baking sheet for 10 minutes, then remove to a wire rack.
Make glaze while cooling by mixing the powdered sugar with milk until the desired consistency.
While the turnovers are warm, but not hot drizzle the glaze icing over the top.
Lastly, enjoy!