Preheat the oven to 200°C.
Wash and stone the peaches, cutting each into 8 slices.
Place the nectarines into a mixing bowl along with the cornflour, caster sugar and vanilla extract, mixing well.
Grease a baking tray and unroll the puff pastry onto it.
Pile the nectarines into the middle (in a rectangle) leaving about a 6cm border of pastry all the way round.
Fold the sides of the pastry up towards the middle (working your way round in sequence).
Brush the pastry with egg wash and sprinkle on the Demerara sugar.
Bake on the middle shelf of oven for 30 mins or until pastry is golden brown and baked through. (Turning towards the end is a good idea for an even bake).
Once the Peach Galette is ready, remove from oven and place on a serving dish.
Place a knob of butter on the end of a fork and rub over the exposed peach slices to form a glossy finish.
Serve warm slices with ice cream.