Cube the butter and add to a food processor with the flour and salt.
Pulse the mixture three or four times until it gets to the breadcrumb stage.
Beat the egg and add to the bowl. Pulse twice.
Check the dough with your fingers to see if it sticks together when pressed. Add a little water if required.
Turn the processor on full and within a few seconds the mixture should come together into a ball.
Divide the dough into 2 portions.
Wrap in plastic and chill for 30 minutes.
Sprinkle some flour on a work surface and roll out one portion of the pastry to line the pie dish.
Add the filling and press down with a fork.
Roll out the pastry lid and cut out 17 large pastry shapes.
Beat the remaining egg with water and use to stick 10 crowns around the edge.
Repeat with another layer of 7 crowns and egg wash, slightly overlapping the first.
Complete the pie by rolling out the remaining dough and cutting 9 smaller crowns.
Cover the whole surface in the egg wash.
Preheat the oven to 190 C / 375 F / 170 FAN / Gas 5.
Bake for about 30 minutes until crisp and golden.
Allow the pie to rest for 5 minutes before serving.