Prepare either pie crust recipe through step 5. Keep both dough discs chilled so they roll smoothly without becoming soft or greasy.
Combine the blueberries, sugar, flour, cornstarch, cinnamon, lemon juice, and lemon zest in a large bowl. Stir until no dry coating remains, crushing a few berries only if needed to moisten the mixture; cover and refrigerate the filling for up to 24 hours if needed.
Set an oven rack in the lower third of the oven and heat the oven to 425°F (218°C). Place a baking sheet on the bottom rack to catch any bubbling berry juices.
Roll one chilled dough disc on a floured surface into a 12-inch circle, rotating it a quarter turn after every few rolls, then fit it into a 9 x 1.5 or 2-inch round pie dish. Press the dough gently into the bottom and sides without stretching it, then add the filling evenly and scatter the butter cubes over the berries.
Roll the remaining chilled dough into another 12-inch circle and cut it into strips with a pastry wheel, sharp knife, or pizza cutter. Weave the strips over and under the filling, seal them to the bottom crust, trim the excess, and crimp or flute the edge until it feels firmly joined.
Brush the lattice and rim lightly with the egg wash. Sprinkle with coarse sugar, if using. The egg wash should leave a thin, even sheen rather than puddles between the pastry strips.
Bake for 25 minutes at 425°F, then reduce the oven temperature to 375°F (190°C) without removing the pie and shield the edges with a pie crust shield or aluminum foil. Continue baking for 40–50 minutes, until the crust is deep golden and juices boil vigorously across the center; the filling should reach around 200°F (93°C) on an instant-read thermometer.
Transfer the pie to a cooling rack and leave it undisturbed for at least 4 hours before cutting. The filling thickens from loose bubbling juice into a settled, glossy jam as it cools.
Cover leftover pie tightly and store it in the refrigerator for up to 5 days. Keep the crust protected from moisture so the lattice stays as crisp as possible.