Roll out the puff pastry sheet. If yours is round like mine, cut out a 18cm x 25cm rectangle.
Slightly fold the edges of the rectangle and using a fork press evenly around the entire outer edge of the pastry. Use a fork to pierce the pastry in several places, every 2 cm or so.
Core and peel the apples. Cut each apple in half and then slice as thinly as possible.
Throw the slices into a bowl and squeeze some lemon juice over them.
Beat the egg and brush the pastry with it.
Arrange the apple slices in a straight line on top of the pastry ,overlapping them as you go.
In a cup mix sugar and butter, heat them in a microwave until melted. Brush the apple slices with the mixture, then sprinkle with cinnamon.
Bake at 180°C for about 25-35 minutes depending on your oven. The pastry must sufficiently puff and come out golden brown.