Place the flour, sugar, and salt in the bowl of a food processor using the steel blade. Pulse a few times to combine. Add the cold butter to the food processor and pulse 12 to 15 times, or until the butter is broken down and the size of peas. With the motor running, add the ice water through the feed tube. Continue to pulse until the water is combined, be sure not to overwork it. Place the crumbly dough onto a well-floured work surface worked into a ball and form into flat disks (one large or two small.) Wrap the disks tightly in plastic so no air gets in and refrigerate for at least 1 hour.

Place your blueberries in a bowl with cornstarch, sugar, lemon juice, and zest. Using your hands gently combine. Set aside.

Heat your oven to 375 degrees.
Flour your work surface and using a floured rolling pin, roll out your pastry. Rotating as you go. Until you have a large disk. Transfer to a baking sheet covered in parchment paper. Place the blueberries in the center of the pastry and fold the border of pastry over the fruit, pleating as you go.
Using a fork whisk the egg in a small bowl with 1/2 tablespoon of water. Brush the egg wash on the outside edge of the pastry and sprinkle with sugar. Bake in the oven for about 40 minutes until the pastry is golden and the fruit is bubbling. Enjoy.