For crust: Combine flour, sugar, cinnamon and salt in medium bowl. Cut in cold butter using pastry blender until mixture resembles coarse meal. Add water and toss mixture gently with a fork until evenly moistened. Gather gently into ball.
Transfer to lightly floured board and roll into circle slightly larger than a deep 10-inch pie plate. Ease pastry into pan and flute a high edge. Set aside.
For filling: Preheat oven to 450*F. Combine apples, sour cream, sugar, flour, egg, vanilla and salt in large bowl and mix well. Spoon into crust. Bake 10 minutes. Reduce oven temperature to 350*F and continue baking until filling is slightly puffed and golden brown, about 40 minutes. (If edges of crust begin to brown too quickly, cover with strips of aluminum foil.).
For topping: Meanwhile, combine walnuts, flour, sugars, cinnamon and salt in medium bowl and mix well. Blend in butter until mixture is crumbly. Spoon over pie and bake 15 minutes longer.