Preheat your oven to 400°F (200°C).
Mix the Filling: In a large bowl, toss blueberries with sugar, cornstarch, lemon juice, zest, cinnamon, and salt until everything is evenly coated.
Prepare the Crust: Roll out one pie crust and fit it into a 9-inch pie dish. Pour in the blueberry mixture and dot with butter.
Top It Off: Cover with the second crust—feel free to get creative with a lattice or keep it classic. Trim and crimp the edges.
Egg Wash: Whisk together the egg and milk, then brush over the top crust for a golden finish.
Bake: Place pie on a baking sheet (to catch drips) and bake 45-50 minutes, until the crust is golden and the filling is bubbling. If the edges brown too quickly, cover them with foil.
Cool: Let the pie cool for at least 2 hours before slicing—this helps the filling set.