Cook the apples, sugar, butter and cinnamon in a medium pan over medium heat until the butter has melted. Increase heat to medium-high and bring to a simmer. Cook until the apples have softened and the liquid reduced, around 10min, stirring occasionally. Decant into a small bowl and set aside to cool to room temperature.  Preheat oven to 200°C (180°C fan) mark 6 and grease and line a large baking sheet with baking parchment. Unroll the pastry sheet and gently roll out, if necessary, so it is roughly 23 x 35cm. Cut the pastry sheet into 4 even rectangles.  Working with 1 pastry rectangle at a time, with a long edge closest to you, spread 11/2tbsp Biscoff over the right 1/2 of each rectangle, leaving a 1cm border at the pastry edges. Spoon 1/4 of the cooled apple mixture over the Biscoff and smooth to level with the back of a spoon. Repeat with remaining pastry, Biscoff and apple. Lightly brush the pastry borders around the filling with egg. Fold pastry over the filling, short side to short side, then crimp pastry edges with a fork to seal. Transfer to the prepared baking sheet, spacing apart. Brush the top of each pastry pocket with egg and sprinkle over the demerara sugar. Using a sharp knife, cut three small slashes in the top of each pastry pocket. Bake for 35-40min until risen and the undersides are golden. Cool on the baking tray for 15min before serving.