Preheat your oven to 200°C (400°F).

In a bowl, mix together the diced apples, sugar, cinnamon, and lemon juice.

Roll out the puff pastry sheet and cut it into 4 squares.

Place a spoonful of the apple mixture onto each square.

Fold the pastry over the filling to create a triangle shape and press the edges to seal.

Brush the turnovers with beaten egg.

Bake in the preheated oven for 20-25 minutes, or until golden brown.

Allow to cool slightly before serving.