In a large bowl, whisk together the flour, sugar, and salt. Add the cold butter cubes and use a pastry cutter or your fingertips to work the butter into the flour until the mixture resembles coarse crumbles with some pea-sized butter pieces remaining. Do not overwork.

Drizzle in 6 tablespoons of ice water, one tablespoon at a time, tossing gently with a fork after each addition until the dough just begins to hold together when pinched. Add up to 2 more tablespoons of ice water if needed. The dough should not be wet or sticky.

Divide the dough into two equal portions and shape each into a flat disc. Wrap each disc tightly in plastic wrap and refrigerate for at least 1 hour, or up to 3 days.

In a large bowl, combine the fresh blueberries, granulated sugar, lemon juice, lemon zest, cinnamon (if using), and salt. Stir gently and let sit for 10 minutes to allow the berries to release some of their juices.

Sprinkle the cornstarch over the berry mixture and stir until fully incorporated and no white streaks remain. Set aside while you roll out the crust.

Preheat your oven to 425°F (220°C). Place a rimmed baking sheet on the lower oven rack to preheat.

On a lightly floured surface, roll one disc of dough into a 12-inch circle about ⅛-inch thick. Carefully transfer it to a 9-inch pie dish, pressing it gently into the bottom and sides. Trim any overhang to about 1 inch beyond the rim. Refrigerate while you prepare the lattice.

Roll the second disc of dough into a 12-inch circle. Using a sharp knife or pastry wheel, cut the dough into strips about ¾-inch to 1-inch wide. You will need about 10-12 strips. Transfer the strips to a parchment-lined baking sheet and refrigerate for 10 minutes.

Pour the blueberry filling into the chilled bottom crust and spread evenly. Dot the top of the filling with the small pieces of butter.

Weave the lattice: Lay half of the strips across the pie, evenly spaced, in one direction. Fold back every other strip halfway. Lay a strip perpendicular across the unfolded strips, then unfold the folded strips back over it. Fold back the alternate strips, place another perpendicular strip, and unfold. Continue until the lattice covers the entire pie.

Trim the lattice strip ends to align with the bottom crust overhang. Fold and crimp the edges together all the way around to seal. Use your fingers or a fork to create a decorative crimp.

Whisk together the egg and milk to make the egg wash. Brush it evenly over the entire lattice top and crimped edges. Sprinkle generously with turbinado sugar.

Place the pie on the preheated baking sheet in the oven. Bake at 425°F for 20 minutes, then reduce the oven temperature to 375°F (190°C). Continue baking for an additional 30-35 minutes, until the crust is deep golden brown and the filling is bubbling vigorously through the lattice. If the crust edges begin to brown too quickly, cover them loosely with strips of foil or a pie shield.

Remove the pie from the oven and transfer to a wire rack. Allow the pie to cool completely at room temperature for at least 3-4 hours before slicing. This is essential for the filling to fully set.