Roll out the pastry on a lightly floured surface to about a 30 cm (12 inch) round and place on to a large baking sheet. Sprinkle over the ground almonds, then pile the nectarines in the centre of the pastry.
Brush the edges of the pastry with a little of the beaten egg, then bring the pastry edges into the centre, leaving the fruit exposed in the middle. Scrunch up the edges of the pastry, then brush with more beaten egg. Sprinkle over the sugar and flaked almonds.
Place in a preheated oven, 200ºC (400ºF), Gas Mark 6, for 20–25 minutes or until the pastry is browned.
Dust the warm pie with icing sugar and serve in slices with double cream, if using.