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Put a large baking sheet in middle of oven and preheat oven to 450°F.
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Whisk together granulated sugar, cornstarch, tapioca, and salt, then toss with berries.
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Roll out 1 piece of dough (keep remaining piece chilled) on a lightly floured surface with a lightly floured rolling pin into a 13-inch round and fit into pie plate. Trim edge, leaving a 1/2-inch overhang. Chill shell while rolling out dough for top crust.
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Roll out remaining piece of dough on lightly floured surface with floured rolling pin into an 11-inch round.
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Spoon filling into shell, then cover pie with pastry round and trim with kitchen shears, leaving a 1/2-inch overhang. Press edges together, then crimp edge decoratively. Brush top of pie with egg and sprinkle all over with sanding sugar. Cut 3 steam vents in top crust with a small sharp knife.
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Bake pie on hot baking sheet in middle of oven 15 minutes, then reduce oven temperature to 375°F and continue to bake until crust is golden brown and filling is bubbling, about 45 minutes more.
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Cool pie on a rack at least 3 hours before serving to allow juices to thicken slightly (filling will still be juicy).
