To a medium saucepan, add the chopped apple, sugar, and cinnamon and stir well to coat the apple. Put the saucepan over medium heat, cover it, and cook for 5 minutes, stirring occasionally. Stir in the flour and cook 30 seconds more, stirring constantly. Cool to room temperature. You should have about 1 cup of apple filling.
ASSEMBLE AND BAKE THE TURNOVERS:
Preheat the oven to 400 degrees F. Line 2 large baking trays with parchment paper or silpat liners.
Unfold the puff pastry sheets, and roll them both out on a lightly floured surface to a square about 10 inches. Cut each sheet into 4 equal parts. Place 2 tablespoons of apple filling in the center of each of the 8 puff pastry squares. Lightly brush 2 adjacent sides of each pastry square with eggwash, and fold them over onto the non-eggwashed edges to form triangles.
Arrange the pastries on the prepared baking trays. Lightly brush the tops with more eggwash. Crimp the sides of each pastry with a fork. Use a paring knife to cut 3 small slits in the top of each pastry. Bake until the pastries are puffed and golden, about 18 to 25 minutes minutes, rotating the trays once halfway through. Cool completely before glazing.
GLAZE THE TURNOVERS:
To a medium bowl, add the powdered sugar and vanilla. While stirring with a fork, add enough milk so that it forms a glaze consistency. Transfer the pastries to a wire rack. Drizzle the glaze on top. Let the glaze set before serving.