In a large bowl, whisk together the flour, granulated sugar, and salt for the crust. Add the very cold, cubed butter and cut it into the flour until it resembles coarse crumbs. Gradually add ice water until the dough just comes together. Form into a disk, wrap in plastic, and chill for at least 30 minutes.

In a medium bowl, combine blueberries, granulated sugar, flour (or cornstarch), lemon zest, lemon juice, vanilla extract, and a pinch of salt. Toss until blueberries are coated and set aside.

Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper. Roll out the chilled dough into a rough 12-14 inch circle on a floured surface. Transfer to the baking sheet.

Spoon the blueberry filling onto the center of the dough, leaving a 2-inch border. Fold the edges of the dough over the filling, pleating as you go.

Whisk together the egg and water for the egg wash. Brush the crust with the egg wash and sprinkle turbinado sugar over it.

Bake for 35-45 minutes until the crust is golden brown and the filling is bubbly. If the crust browns too quickly, tent with foil.

Let cool for 15-20 minutes before slicing. Serve warm, optionally with vanilla ice cream or whipped cream.