Preheat the oven to 375°F. Line the baking sheet with parchment paper.
Roll the puff pastry to a thickness of about 3 mm. Use a knife to cut it into 8 equal squares.
Add butter in a pan and melt over medium-low heat.
Add diced apples, flour, sugar, and cinnamon. Stir well and cook over medium heat for a couple of minutes until the apples are tender. Remove from heat and let it cool.
Add a teaspoon of apple mixture in the center of each square.
Brush the edges of each square with water and fold pastry to form a triangle. Press gently the edges. Cut 3 air vents in each triangle and place them on the baking sheet.
Refrigerate the triangles for about 15 minutes.
Bake for 30 minutes, until the pastry is golden brown.
You can prepare white glaze with powdered sugar and milk, and drizzle it over apple turnovers. You can also mix in edible colors.
For a crunchy texture, you can add in some chopped nuts like walnuts, almonds, or pecans.