In a large bowl, whisk together the flour, sugar, and salt. Add the cold cubed butter and use a pastry cutter or your fingertips to work it into the flour until the mixture resembles coarse crumbs with some pea-sized butter chunks remaining — those chunks are what create flaky layers.

Drizzle in 6 tablespoons of ice water, one tablespoon at a time, tossing with a fork after each addition. Stop adding water when the dough just begins to hold together when you pinch a piece. It should not be wet or sticky.

Turn the dough out onto a lightly floured surface and gently bring it together. Divide into two equal discs, wrap each in plastic wrap, and refrigerate for at least 1 hour or up to 2 days.

In a large bowl, combine the blueberries, granulated sugar, tapioca starch, lemon juice, lemon zest, vanilla extract, cinnamon, and salt. Stir gently until the berries are evenly coated. Let the mixture sit for 15 minutes while you roll out the dough.

Preheat your oven to 425°F (220°C) with a rack positioned on the lowest level. On a lightly floured surface, roll one disc of dough into a 12-inch circle. Carefully transfer it to a 9-inch pie plate, pressing gently into the bottom and up the sides. Trim the overhang to about 1 inch and refrigerate while you roll the second disc.

Roll the second dough disc into a 12-inch circle. Using a sharp knife or pastry wheel, cut it into strips about 3/4 to 1 inch wide for the lattice.

Pour the blueberry filling into the chilled bottom crust, spreading evenly. Lay half the dough strips across the filling in one direction, evenly spaced. Fold back every other strip halfway, then lay a perpendicular strip across the center. Unfold the folded strips back over, then fold back the alternating strips to lay another perpendicular strip. Continue weaving in this manner until the entire pie is latticed.

Trim the strip ends to match the overhang of the bottom crust, then fold the bottom crust edge up and over the strip ends, pressing to seal. Crimp the edge decoratively with your fingers or a fork.

Brush the entire top crust and crimped edges generously with the egg wash. Sprinkle evenly with coarse sugar.

Place the pie on the lowest oven rack. Bake at 425°F for 20 minutes, then reduce the oven temperature to 375°F (190°C) and continue baking for 35-40 more minutes, until the crust is deep golden brown and the filling is visibly bubbling through the lattice. If the edges brown too quickly, tent them loosely with strips of foil.

Remove the pie from the oven and place it on a wire rack. Allow it to cool for at least 2-3 hours before slicing — this resting time is essential for the filling to fully set and slice cleanly. Serve at room temperature or slightly warm with vanilla ice cream.