Heat oven to 180C/fan 160/gas 4 and grease a large baking sheet. Unravel the pastry and place on the baking sheet. Mix the sliced apple and rhubarb with the sugar, ginger and cornflour then pile into the centre of the pastry. Gather up the sides of the pastry to enclose the fruit so that the pie looks like a rough tart - you need to work with the size of the pastry so it will be more of an oblong shape than round.
Brush the pastry with milk and scatter with demerara. Bake for 35 mins until the pastry is golden and the fruit is tender. Cut into slices and serve with custard.