Preheat oven to 375°F (190°C).
Roll out the sourdough pie crust and fit it into a 9-inch pie pan.
In a large bowl, combine blueberries, sugar, flour, cornstarch, cinnamon, lemon juice, and lemon zest. Toss gently to coat.
Pour the blueberry mixture into the prepared pie crust.
Place the pie on a baking sheet to catch any drips.
In a small bowl, beat the egg with a splash of milk to create an egg wash.
Brush the edges of the pie crust with the egg wash.
Optionally, add a lattice top or second crust and brush with egg wash as well.
Bake the pie for 45-50 minutes, or until the crust is golden and the filling is bubbly.
If the edges brown too quickly, cover with foil halfway through baking.
Remove from the oven and let cool for at least 2 hours before serving to allow the filling to set.