Toss the blueberries in a large bowl with the cornstarch, sugar and lemon juice.
On a lightly floured sufrace, roll out one disc of pie dough into a 12-inch circle. Roll the circle around your rolling pin, then transfer to a 9-inch pie pan or dish by unrolling it over the pan. Gently press the dough into the bottom and up the sides, letting the excess hang over the edge of pan. Place in the refrigerator while you roll out the other half of the dough
Roll out the remaining dough into a 12-inch circle.
Add the blueberries to the pie pan. Place the second dough circle on top of the berries. Press the dough together around the edge of the pan to seal. Gently press a rolling pin over the edge of the pan to trim off any overhanging dough. If you like, crimp the edges or press with the tines of a fork to decorate. Or just leave it alone.
Use a small, sharp knife to cut 5 or 6 4-inch-long slits around the top of the pie (this helps steam to escape as the pie bakes).
Brush the egg over the top of the pie and sprinkle with sugar.
Reduce the oven temperature to 375. Place the pie pan on a baking sheet and bake 60- 70 minutes, until the crust is deep golden on top and the juices are bubbling.
Cool completely before serving (at least 2 hours, and preferably 4).