Preheat the oven to 180˚C / 350˚F.
Unroll the puff pastry sheet, straight onto a baking tray, keeping it on the baking parchment it comes in, to bring it to room temperature.
Dice the apples into small squares.
Melt the butter in a saucepan on a low-medium heat. Add the light brown sugar, cinnamon and apples and cook, stirring regularly for 8-10 minutes until the apples have softened but are still holding their shape. Add a splash of water if needed.
Remove pan from heat and allow apple mixture to cool.
Cut the pastry sheet into four rectangles.
Place a spoonful of the cooled apple filling in the left hand side of each rectangle, leaving a border of pastry around the edges.
Brush the edges of the pastry squares with the beaten egg, then fold the pastry over the filling to create a rectangle shape.
Press the edges of the pastry together to seal the turnovers, then use a fork to crimp the edges.
Brush the top of each turnover with more beaten egg and sprinkle with granulated sugar.
Use a sharp knife to cut a few small slits in the top of each turnover to allow steam to escape during baking.
Place the turnovers in the pre-heated oven and bake for 21-24 minutes, or until the pastry is golden brown and puffed up.
Remove from the oven and let cool for a few minutes on a wire rack before serving.