INGREDIENTS
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1/2 stick unsalted butter
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3 pounds Granny Smith apples (6 to 7), peeled, cored, and cut into eighths
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1 tablespoon fresh lemon juice
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1/4 cup all-purpose flour, plus more for rolling
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1/2 teaspoon coarse salt
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3/4 cup granulated sugar
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1/2 cup packed light-brown sugar
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1 vanilla bean, seeds scraped
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Double-Crust Pie Dough
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Water, for brushing
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1 large egg, lightly beaten
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Coarse sanding sugar, for sprinkling