In a large bowl, whisk together flour, sugar, and salt. Add cold butter cubes and cut in with a pastry cutter until mixture resembles coarse meal with some pea-sized butter pieces remaining. These butter pockets create flakiness.

Drizzle 6 tablespoons ice water over mixture and toss with a fork until dough just starts to hold together. Squeeze a small handful - if it crumbles, add more water 1 teaspoon at a time. Divide dough in half, shape into two 1-inch thick disks, wrap tightly, and refrigerate at least 2 hours or up to 2 days.

In a medium saucepan, combine 3 cups blueberries with sugar, cornstarch, lemon zest, lemon juice, and cinnamon. Let sit 10 minutes until berries release some juice, then cook over medium heat, stirring constantly, until mixture thickens and bubbles vigorously, about 6-8 minutes. It should coat the back of a spoon.

Remove from heat and stir in remaining 3 cups fresh blueberries. This gives you the best of both worlds - jammy base with whole berries that burst in the oven. Let cool to room temperature, about 30 minutes.

On a floured surface, roll one dough disk to a 12-inch circle about 1/8-inch thick. Transfer to a 9-inch pie plate, gently pressing into corners without stretching. Trim overhang to 1/2 inch. Refrigerate while you roll the top crust.

Pour cooled filling into the chilled bottom crust. Roll second dough disk to 11 inches and place over filling. Trim, fold top edge under bottom edge, and crimp with fingers or a fork. Cut 6 steam vents in the center. Brush with egg wash and sprinkle with coarse sugar.

Place pie on a rimmed baking sheet to catch drips. Bake at 425F for 20 minutes, then reduce to 375F and bake 35-40 minutes more until crust is deep golden and filling bubbles thickly through the vents. If edges brown too quickly, cover with foil or a pie shield. Cool completely on a wire rack, at least 4 hours, before slicing - the filling needs time to set.