Preheat the oven to 220C/200C Fan/Gas 7 and line a baking tray with baking paper.
Place the apples, light brown sugar, cinnamon, ginger, butter and lemon juice in a saucepan and cook for 10 minutes until softened slightly. Transfer to a bowl and leave to cool. Once cool, stir in the cornflour.
Unroll the pastry on its baking paper and use a sharp knife to cut into six equal-sized squares. Fill each square with 2 tablespoons of the apple filling, leaving a 1cm/½in border around the outside.
Brush the border with the beaten egg and fold the squares diagonally in half to create a triangle shape. Press down the edges to seal the filling and crimp with a fork all around.
Brush the triangles with the remaining beaten egg and sprinkle the demerara sugar over the top.
Place the paper containing the pastries on to the baking tray, spacing them well apart. Make a small incision with a knife in the top of each turnover to allow steam to escape. Bake for 20–25 minutes until golden brown.
Eat warm or leave to cool completely on the tray before eating.
