Heat oven to 400 degrees F. Unroll pie crust in 9-inch glass pie plate as directed on box for One-Crust Filled Pie. In large bowl, beat sour cream, granulated sugar, flour, salt, vanilla and egg with wire whisk until well blended; stir in apples. Pour into crust-lined pie plate. Bake 15 minutes. Reduce oven temperature to 350 degrees F; bake 30 minutes longer. Meanwhile, in medium bowl, mix all topping ingredients except butter. With pastry blender or fork, cut in butter until mixture looks like coarse crumbs; refrigerate until ready to use. Sprinkle topping over pie. Bake 20 to 25 minutes longer or until topping is golden brown. Cool completely on cooling rack, about 2 hours. Cover and refrigerate any remaining pie. High Altitude (3500-6500 ft): In step 4, bake 25 to 30 minutes.