Apple-Pear Pie with Walnut Crust

INGREDIENTS

• 2 cups all-purpose flour, plus more for surface
• 1 1/2 ounces walnuts, toasted and chopped (1/2 cup)
• 1 tablespoon granulated sugar
• 1 teaspoon coarse salt
• 2 sticks cold unsalted butter, cut into small pieces
• 1/3 to 1/2 cup ice water
• 5 to 6 Granny Smith apples
• 3 firm-ripe Bosc pears
• 1/4 teaspoon salt
• 2/3 cup granulated sugar
• 1 whole vanilla bean, split and scraped
• Juice of 1/2 lemon (about 5 teaspoons)
• 1/2 stick unsalted butter
• 3 tablespoons all-purpose flour
• 1 large egg, lightly beaten
• Fine sanding sugar