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 pea-sized butter pieces throughout. Do not overwork.

Drizzle in the ice water one tablespoon at a time, mixing gently with a fork after each addition, until the dough just comes together when you squeeze a small handful. It should not be sticky or wet.

Turn the dough out onto a lightly floured surface, divide into two equal discs, wrap each tightly in plastic wrap, and refrigerate for at least 1 hour (or up to 3 days).

In a large bowl, combine the blueberries, sugar, cornstarch, lemon juice, lemon zest, and salt. Toss gently until the berries are evenly coated. Set aside and let the mixture sit for about 15 minutes while you roll out the dough — this allows the berries to release some juice.

Preheat your oven to 425°F (220°C) with a rack positioned in the lower third of the oven. Line a rimmed baking sheet with foil and set aside.

On a lightly floured surface, roll out one disc of dough into a 12-inch circle about ⅛-inch thick. Carefully transfer to a 9-inch pie plate, gently pressing it into the bottom and sides. Trim any overhang to about ½ inch. Refrigerate while you roll out the second disc.

Roll out the second disc of dough into a 12-inch circle. Using a sharp knife or pastry wheel, cut it into 10 to 12 strips, each about ¾ to 1 inch wide.

Pour the blueberry filling into the prepared crust, spreading evenly. Dot the top of the filling with the small pieces of cold butter.

Weave the lattice: lay half the strips across the filling in one direction, evenly spaced. Fold back every other strip halfway, lay a perpendicular strip across, unfold the folded strips back down, then fold back the alternate strips and lay the next perpendicular strip. Continue until the lattice covers the pie. Trim excess and crimp the edges firmly with your fingers or a fork to seal.

Brush the lattice and crimped edges all over with the egg wash. Sprinkle generously with coarse sugar.

Place the pie on the foil-lined baking sheet (to catch drips). Bake at 425°F for 20 minutes, then reduce the oven temperature to 375°F (190°C) and continue baking for 35 to 40 minutes more, until the crust is deep golden brown and the filling is actively bubbling through the lattice vents.

If the crust edges are browning too quickly, loosely tent them with strips of foil or a pie shield after the first 20 minutes.

Transfer the pie to a wire rack and allow it to cool for at least 3 to 4 hours before slicing. This step is essential — the filling needs time to fully set so it slices cleanly rather than pouring out. Serve at room temperature with vanilla ice cream or softly whipped cream.