Roll out the puff pastry to about ⅛ inch thick and cut into a circle using a plate or lid as a guide.
Crimp the edges of the pastry by rolling and folding it back and forth.
Lift the pastry circle and place it on a baking surface, then chill in the fridge for 30 minutes.
Peel and slice the Granny Smith apples, removing the cores.
Layer the bottom of the pastry with a thin layer of apple sauce, leaving about an inch from the edge.
Arrange the apple slices in a spiral pattern on top of the apple sauce.
Sprinkle 2 teaspoons of castor/superfine sugar over the apples.
Brush the pastry edges with eggwash.
Dot the top of the apples with 50g/2oz of butter.
Bake the tart at 200°C/400°F for 30 minutes.
