Make the pie dough: Combine flour, salt, and granulated sugar in a food processor, and pulse to blend. Add butter, pulsing until pea-size clumps form. With machine running, add water, 1 tablespoon at a time, until mixture just comes together. Shape dough into a disk. Wrap in plastic wrap, and refrigerate until firm, about 45 minutes.
Roll out dough into a 15-inch round (1/4 inch thick) on a lightly floured surface. Fit into a 10-inch pie plate. Trim edge, leaving a 1/2-inch overhang, and fold under, pressing onto rim of pie plate. Use your fingers to create a scalloped edge if desired. Refrigerate until firm, about 1 hour.
Make the crumb topping: Whisk together sugars, flour, cinnamon, and salt. Add butter, and combine with a fork or pastry cutter. Stir in walnuts, and press filling into large clumps with your hands; refrigerate until ready to use.
Preheat oven to 450 degrees with 2 racks in lower third.
Make the filling: Whisk together granulated sugar, flour, cinnamon, and salt in a large bowl. Stir in sour cream, eggs, and vanilla until thoroughly combined. Add apples, tossing to coat. Place apple mixture in crust.
Place an empty rimmed baking sheet on lowest oven rack to catch any drips, then bake pie 10 minutes. Reduce oven temperature to 350 degrees, and bake until apples are golden brown in places, juices are bubbling, and crust is golden brown, about 45 minutes.
Remove topping from refrigerator, and crumble over hot filling. Bake until topping is browned and set, apples are tender, and bottom crust is thoroughly baked, about 50 minutes. Let cool completely on a wire rack, 3 to 4 hours.