For the crust:
Combine flour, sugar, and salt in large bowl. Add butter and mix using pastry blender or a fork. Mix until largest pieces of butter are about the size of large peas.
Add water, about a tablespoon at a time, and mix just until dough forms.
Divide dough in half. Flatten each into a disc about an inch thick. Wrap each disc tightly in plastic wrap. Refrigerate for at least 2 hours.
Place one chilled disc of dough on lightly floured surface. Roll dough into a circle about 1/8-inch thick.
Transfer the dough to 9-inch round pie plate. Trim and crimp the edges as desired. Place pie plate in refrigerator while you prepare filling.
For the filling:
Preheat oven to 350°F. Line a rimmed baking sheet with parchment paper.
Stir together blueberries, sugar, cornstarch, ginger, zest, and juice. Set aside.
Place the remaining disc of pie dough on lightly floured surface. Roll dough into a circle about 1/8-inch thick. Use a small cookie cutter (or a combination of sizes) to cut out circles of dough.
Pour blueberry mixture into chilled pie crust. Scatter pieces of butter over top of the filling. Top with dough circles, arranging to cover most of the top while leaving some uncovered spots for the filling to vent.
Whisk together egg and water to make an egg wash. Lightly brush edge of pie crust and dough circles with egg wash. (You won’t use all of the egg wash.) If desired, sprinkle top of pie with sanding sugar to give the pie a bit of sparkle.
Place pie plate on lined baking sheet to catch any spills. Bake 55 to 60 minutes, or until crust is golden brown and filling is bubbly. Let pie cool completely before serving.