Take 8 3/4 oz 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 50 g butter in a large frying pan over a moderate heat, add the apples and cook gently for 5 minutes. Stir in the sugar and cinnamon, remove from the heat and allow to cool completely.

Heat the oven to 190°C/gas 5.

Dust a baking sheet with flour and roll out the pastry to a circle about 1cm 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.

Bake for 20 minutes. Reduce the heat to 160°F/gas 3 and bake for 20 minutes more.