In a food processor, blend together the flour, salt and sugar.
Add the butter and pulse into your have a sandy looking mixture.
Add 1/8 cup of ice water and pulse a few more times. If it looks too dry, add the other 1/8 cup one tablespoon at a time. Depending on your food processor, your mixture may or may not come together in a ball. If it doesn't, don't keep pulsing. Just dump onto a counter a form a ball with your hands.
Wrap in plastic and refrigerate for at least 30 minutes. Don't skip this. Don't put it in the freezer thinking it will be done in half that time. This is your dough's time to relax and chill out before being beaten down and baked. Let him have it.
Mix the berries, sugar, zest and cornstarch together. Refrigerate until you're ready to use.
Rolling out the dough
When your dough is nice and chilled, transfer it to a lightly floured surface and begin rolling it out to a 13 inch (33 cm) circle.
*Rolling tips - Start from the center always and roll outwards, turn 1/4 way and roll again, turn, roll, turn, roll and so on. Also, keep lifting up the dough and adding more flour as your turn so it doesn't stick.
Transfer the circle of dough to a cookie sheet lined with parchment paper. Dump the berry mixture in the middle and start folding one edge of the dough towards the center. Go around in a circle, pressing the new dough fold lightly into the one under it. Chill the pie for about 10-15 minutes before baking.
While your pie is chilling, preheat your oven to 400 degrees F (200 degrees C). Make your egg wash.
When the pie is chilled and the oven preheated, brush the egg wash on the crust (optional: sprinkle with sugar) and bake for 25 - 30 minutes, until crust is golden brown and berries are bubbling.