Mix topping ingredients in a small bowl using a pastry blender, until mixture resembles coarse pebbles. Set aside in refrigerator until needed.
Mix sour cream, egg, sugar, flour, salt, cinnamon, nutmeg and vanilla in large bowl.
Peel the apples and slice into pieces about 1/4 inch thick. As you slice each apple, add to the bowl with the sour cream mixture and stir to coat apples so that they won't turn brown. Add the walnuts to the mixture and stir.
Place apple mixture in frozen pie shell, and bake in 400° oven for 25 minutes
After 25 minutes of baking, remove pie from oven and top with topping mixture. Return to oven and bake at 400° for an additional 20 minutes. Let cool one hour before cutting.