Lower your oven rack one spot down from the middle rack. Preheat your oven to 400 °F and get out a 9-inch pie plate.
Place the blueberries in a large bowl. Whisk together the sugar, cinnamon, nutmeg and cornstarch. Sprinkle the mixture over the blueberries in the bowl, tossing to ensure that the blueberries are coated. Set aside and let it meld together while you prepare the crust.
Take one pastry disk and place it on a floured work surface. Flour your rolling pin, then roll the pie dough into a circle that is around 2 inches wider than your pie plate. When rolling, turn the pie dough on the counter every two rolls or so, so that it doesn't stick in one place to the counter, then roll again.
To place in the pie plate, gently fold the pastry into a half-circle, then fold again.
Carefully place in the pie plate. Unfold gently and let gravity help ease into the pie plate instead of you pulling and pushing on it. Once it's settled, press firmly against the bottom and sides.
Place the blueberry mixture into the pie crust bottom. Sprinkle with lemon juice and then dot with butter. Roll out the next pastry disk like the other one. Cut the pastry into 3/4 inch wide strips and make a lattice top. Crimp and flute the edges.
Prepare the egg wash and brush over top of the top pie crust. Sprinkle generously with the sanding sugar, making sure it's a nice even layer on all of the lattice pieces.
Place the pie in the preheated oven and turn the oven down to 375°F.
Bake for 40-45 minutes or until the crust is golden brown and the blueberry filling is bubbling.
If the lattice is getting too golden brown before the filling is bubbling, tent some tin foil over the pie loosely and continue to bake.
Remove from the oven and cool on a baking rack for 2-3 hours to let the filling set.
Serve topped with vanilla ice cream or whipped cream!
Notes
Blueberry size matter! Use large blueberries for a chunky pie, or smaller ones for a pie with more juice in it.