Make 1 recipe quick puff pastry. Keep in the freezer for 15-30 minutes depending on how hot the weather is. The butter in the dough should be cold when it is time to roll the dough in order to have flaky layers in your pastry. It should be cold but at the same time you should be able to roll it without cracks(if the dough is extremely cold it would be hard to roll the dough.)
Preheat the oven to 390°F(200C°). Line baking sheet with parchment paper or silicone baking mat.
Peel, core and slice the apple or apples into small cubes almost ½ inch (1,25 cm). Place the chopped apples into a medium bowl and mix with lemon juice. Then add the sugar, cinnamon, nutmeg and cornstarch, mix with a spoon. Set aside the bowl.
Coat the work surface, the top of the dough and the rolling pin with flour to prevent the dough from sticking.
Start rolling the dough. While rolling, try to give a square shape from the beginning as you'll roll into 11 inch (28cm)square. If your kitchen is too hot during the rolling, you can put the dough in the freezer for 5 minutes to avoid the butter melting. You can repeat this step as needed.
You can either cut the dough into 4 equal pieces to make 4 large apple turnovers. Alternatively, you can make 16 small pieces. After you cut the dough into 4 equal pieces, you should cut each half diagonally and horizontally so from each 4 large square you'll have 4 small squares.4*4=16 squares. If you like you can make both small and large turnovers: After you cut the dough into 4 equal pieces, keep one side as it is (2 large squares) and cut each other 2 squares into 4 (8 small squares).
Fill each corner of the small squares with almost one tablespoon of apple mixture and fill other 2 squares with almost 3 tablespoons of the mixture. Then fold one corner over the other to make a triangle. Press the edges with a fork so two sides of the dough will seal. Then brush the top with egg wash by mixing egg yolk with 1 teaspoon water to have a golden brown shiny top. Make 3 small slits into the middle top of the dough so the steam can escape while baking. Sprinkle cinnamon sugar on top.
Bake the small apple turnovers for 13-15 minutes and the large apple turnovers for 18-20 minutes until the top of the pastries are golden brown. Let them cool for 5 minutes before eating.