Prepare the pastry. On a lightly floured surface, roll dough to form a 12-inch circle. Ease into a 9-inch pie plate. Trim to 1/2 inch beyond edge of pie plate; fold the extra pastry under. Crimp edge. Set aside.
For filling, in a bowl, stir together the granulated sugar, the 2 tablespoons flour, the salt, and cinnamon. Add the sour cream, egg, and vanilla. Stir to combine.
Place apples in pastry-lined pie plate. Pour filling over apples.
For crumb topping, in a medium bowl, stir together the 1/2 cup flour and the brown sugar. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Spoon evenly over filling. To prevent overbrowning, cover edge of the pie with foil. If you like, place on a baking sheet to catch drips.
Bake in a 375 degree F oven 30 minutes; remove foil. Bake for 35 to 40 minutes more or until apples are tender. Remove from oven; cool on a wire rack. Serve warm or at room temperature. Store, covered, in refrigerator. Makes 10 servings.
Stir together flour and salt. Using a pastry blender cut in shortening until pieces are pea-size. Sprinkle 1 tablespoon of the water over part of the mixture; gently toss with a fork. Push moistened dough to side of the bowl. Repeat moistening dough, using 1 tablespoon of the water at a time, until all the dough is moistened. Form dough into a ball. On a lightly floured surface, use your hands to slightly flatten dough. Roll dough from center to edge into a circle about 12 inches in diameter.