Step 1: Make the Pie Crust
In a large bowl, whisk together flour, sugar, and salt. Add cold, cubed butter and use a pastry cutter or fork to work the butter into the flour until the mixture resembles coarse crumbs with pea-sized butter pieces remaining. Drizzle in ice water, 1 tablespoon at a time, mixing gently with a fork until the dough just comes together. Divide dough in half, shape each half into a disk, wrap in plastic wrap, and refrigerate for at least 1 hour.

Step 2: Prepare the Filling
In a large bowl, gently combine blueberries, sugar, tapioca starch, lemon juice, lemon zest, salt, and cinnamon (if using). Toss gently to coat the berries evenly. Let the mixture sit for 15 minutes to allow the tapioca to begin absorbing the juices.

Step 3: Roll Out the Bottom Crust
Preheat your oven to 400°F (200°C). On a lightly floured surface, roll one disk of dough into a 12-inch circle, about ⅛-inch thick. Carefully transfer to a 9-inch pie dish, gently pressing into the bottom and sides. Trim excess dough, leaving about a 1-inch overhang. Refrigerate while rolling out the top crust.

Step 4: Roll Out the Top Crust
Roll the second disk of dough into another 12-inch circle. You can keep it whole for a full top crust, or cut it into 1-inch strips for a lattice design.

Step 5: Assemble the Pie
Pour the blueberry filling into the prepared bottom crust, spreading it evenly. Dot the top of the filling with small pieces of butter. Place the top crust over the filling (or weave your lattice pattern). Trim excess dough and crimp the edges decoratively to seal. Cut 4-5 small slits in the top crust if using a full crust (to allow steam to escape).

Step 6: Apply Egg Wash
Brush the top crust generously with beaten egg and sprinkle with coarse sugar if desired. This creates that beautiful golden-brown finish.

Step 7: Bake
Place the pie on a baking sheet (to catch drips) and bake at 400°F for 20 minutes. Reduce temperature to 350°F (175°C) and continue baking for 35-40 minutes more, until the crust is deep golden brown and the filling is bubbling through the vents or lattice. If the edges brown too quickly, cover them with foil or a pie shield.

Step 8: Cool Completely
Remove from oven and let the pie cool on a wire rack for at least 4 hours before slicing. This cooling time is crucial—it allows the filling to set properly so your slices hold their shape beautifully.