Instructions:

1. Preheat Oven: Set your oven to 400°F (200°C).
2. Prepare Blueberry Filling: In a large bowl, mix blueberries, sugar, cornstarch, cinnamon, salt, and lemon juice until well combined.
3. Assemble the Pie: Roll out the first pie crust and line a 9-inch pie pan with it. Pour the blueberry filling into the crust, and evenly distribute the butter pieces over the top. Cover with the second pie crust, seal the edges, and cut slits in the top for steam.
4. Apply Egg Wash: Brush the top crust with the beaten egg, and sprinkle coarse sugar on top if you like extra crunch.
5. Bake: Bake for 45-50 minutes, until the crust is golden and the filling is bubbling. If the edges brown too fast, cover them with foil.
6. Cool and Serve: Let the pie cool for at least 2 hours before slicing to let the filling set.