Preheat your oven to 375°F (190°C).
Roll out the chilled pie crust and place it into a 9-inch pie dish, crimping the edges as desired.
In a large bowl, toss the fresh blueberries with granulated sugar, cornstarch, and lemon zest until evenly coated.
Pour the blueberry mixture into the prepared pie crust.
In a medium bowl, combine the brown sugar, flour, rolled oats, cinnamon, and salt.
Cut the cold, cubed butter into the crumble mixture using a pastry cutter or your fingers until it resembles coarse crumbs.
Sprinkle the crumble topping evenly over the blueberries.
Whisk the egg and milk together to create an egg wash and lightly brush it onto the exposed edges of the pie crust.
Bake for 45-50 minutes, or until the topping is golden brown and the fruit filling is bubbling.
Transfer to a wire rack and allow to cool completely before slicing to ensure the filling sets.