01 -  In a large bowl, whisk flour, salt, and sugar. Cut in butter with a pastry cutter or fingertips until mixture resembles coarse crumbs. Add ice water 1 tbsp at a time, mixing just until dough holds together. Divide into two disks, wrap in plastic, and chill for at least 1 hour. 
 02 -  In a large bowl, toss blueberries with sugar, cornstarch, flour, lemon juice, zest, cinnamon, and nutmeg. Set aside. 
 03 -  Preheat oven to 400°F (200°C). Roll out one dough disk and line a 9-inch pie dish. Fill with blueberry mixture and dot with butter. Roll out second disk for the top—either a full crust with slits or a lattice. 
 04 -  Brush crust with egg wash and sprinkle with coarse sugar if using. Place pie on a baking sheet and bake for 20 minutes. Reduce heat to 350°F (175°C) and bake for another 30–35 minutes, until filling is bubbly and crust is golden. 
 05 -  Let pie cool for at least 2 hours before serving to allow filling to set.