Step 1: If making homemade crust, prepare your favorite double pie crust recipe and chill it thoroughly for at least 30 minutes. Divide the dough in half, one slightly larger for the bottom crust.
Step 2: On a lightly floured surface, roll out the larger half of the chilled dough into a 12-inch circle. Carefully transfer it to a 9-inch pie plate. Trim the edges, leaving about a 1/2-inch overhang. Place the pie plate with the crust into the refrigerator.
Step 3: In a large bowl, gently combine the fresh blueberries, granulated sugar, lemon zest, lemon juice, cornstarch, and salt. Toss carefully to coat the berries without crushing them.
Step 4: Pour the blueberry filling into the chilled pie crust in the pie plate. Dot the top of the filling with small pieces of unsalted butter.
Step 5: Roll out the second half of the pie dough. Place it over the filling, cut slits for venting, or create a lattice top by cutting dough into 1/2-inch wide strips and weaving them.
Step 6: Trim the excess dough from the top crust, leaving about a 1/2-inch overhang. Crimp the edges of both crusts together to seal, creating a decorative edge. For a golden finish, brush the top crust with the egg wash and sprinkle with coarse sugar, if using.
Step 7: Preheat your oven to 400°F (200°C). Place the pie on a baking sheet. Bake for 20-25 minutes, then reduce the oven temperature to 375°F (190°C) and continue baking for another 35-45 minutes, or until the crust is golden brown and the filling is bubbly. If the crust browns too quickly, loosely tent with aluminum foil.
Step 8: Once baked, remove the pie from the oven. Let it cool completely on a wire rack for at least 3-4 hours (preferably longer, or even overnight) before slicing and serving. This is crucial for the filling to set properly.