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Tip the apples, brown sugar, cinnamon, nutmeg and butter into a saucepan set over a medium heat and simmer, stirring occasionally for 6-8 mins until the apples are soft. Tip the mixture into a bowl and leave to cool.
Meanwhile, unroll the pastry with a short side facing you. Cut in half lengthways so you have two long rectangles, then cut each into five smaller rectangles, so you have 10 in total (they should be 12 x 7cm).
Once the spiced apple mixture has cooled, toss with the cornflour. Spoon into the middle of half the pastry rectangles, leaving the remaining five empty. Brush some of the beaten egg over the exposed pastry around the apples, then carefully place the plain pastry rectangles on top of the filling. Press down around the sides so the filling is encased. Crimp the edges using a fork to seal, then use the tip of a knife to lightly cut three lines across the top of the pies so that steam can escape during cooking.
Arrange the pies on a baking sheet lined with baking parchment and brush all over with more of the beaten egg. Chill for at least 30 mins in the fridge or 10 mins in the freezer. At this stage, the pies can be chilled for up to 24 hrs or individually covered and kept frozen for two months.
Heat the oven to 200C/180C fan/gas 6. Brush with more of the beaten egg, sprinkle with the demerara sugar and bake for 15-20 mins, or until the pastry is golden (add another 10 mins if baking from frozen). Once baked and cooled, will keep in an airtight container at room temperature for up to three days.