In the bowl of a food processor, combine the flour with the salt and sugar. Add the butter and pulse to form a coarse crumble with butter lumps.
Add the water, and pulse to combine. You can continue to run the processor to form a ball of dough or do this on your work surface by hand.
Divide the dough into two disks, wrap in plastic wrap, and chill for 1 hour.
Roll out the first disk of dough to about a 12 inch diameter disk or so on a floured surface with a rolling pin. Transfer to a metal pie plate and make sure to work the dough into the corner edges of the pie plate. Trim the edges and place the pie in the fridge.
Roll out the second disk of dough to a very long rectangle that is quite thin. Cut thin strips of dough, then braid the strips together, three at a time.
In a small bowl, whisk together the egg with 1 tbsp water. Brush the edges of the chilled pie shell with the egg wash, then press the braids to the brushed edges to stick them onto the edge. You can weave the ends of the braids together for a seamless look OR just cut out some leaf shapes with the dough scraps and hide the seams with them, glueing down the shapes with a little more egg wash.
Place the pie shell in the fridge and chill for 20 minutes.