Take 200g of the butter, hard from the fridge, and cut it into very small pieces. Place the pieces on a tray and leave to soften for 20 minutes.
Put the flour in a bowl, add a few pieces of butter and toss them in the flour using a round bladed knife. Continue adding butter until all of it is well coated in flour, so that no pieces get the chance to clump together. Add the cold water and with a cutting motion quickly mix until the pastry comes together into a ball.
Transfer to a lightly dusted, cool surface and gently shape with your hands into a brick. Roll out to an oblong about 18 x 10cm. Fold one third over to the centre, the other third over that, then use the rolling pin to press the edges together, trapping pockets of air. Let it rest for 5 minutes.
Lightly flour the surface again, give the pastry a quarter turn, and repeat as above. Do this three more times. Wrap and chill for 1 hour.
Melt the remaining
Dust a baking sheet with flour and roll out the pastry to a circle about 1 cm thick. Spoon the apples into one half, brush the edges with egg and fold the pastry over into a half-moon shape. Crimp the edges with your fingers to seal them. Brush with egg, pierce with a fork and dust with sugar.