Pre-heat your oven to 375°F and place a rack in the center position. Lightly oil a 9-inch pie dish.
In a small mixing bowl, combine flour, sugar, and cinnamon.
In a large bowl, gently combine the blueberries with the lemon juice. Then add dry ingredients and toss until the blueberries are evenly coated with the mixture.
If you're using a store-bought crust, follow the package instructions for rolling it out and fitting it into a 9-inch pie pan. For a homemade crust, roll out the dough on a lightly floured surface to fit your pie dish, then gently transfer it to the prepared pie dish.
Pour the blueberry filling into the prepared bottom crust, spreading it out evenly.
Spread cubed butter over blueberry pie filling.
Cover the pie with a second pie crust, cutting a few slits in the top to allow steam to escape. Crimp the edges to seal the pie.
Brush the top crust with the beaten egg to create a beautiful golden sheen during baking. Cover edges with aluminum foil or with a pie crust cover, and then transfer it to the preheated oven.
Bake the blueberry pie for 45 to 50 minutes, or until the crust is golden brown and the filling is bubbly. Remove aluminum foil or crust cover about halfway through baking.
Once the pie is done, remove it from the oven. Place on a wire rack and allow to cool for at least one hour before slicing. This allows the filling to set.
Slice a piece and enjoy!