Preheat oven to 400°F. Refrigerate pie shell while preparing filling.
Whisk together sour cream, egg, lemon juice and vanilla. Peel, quarter and core apples, then thinly slice; add slices to sour cream mixture and toss well. Spoon apple mixture into pie shell. Bake pie for 30 minutes, reduce oven temperature to 350°F and bake for 20 minutes.
Melt butter. Stir together flour, brown sugar, granulated sugar, walnuts, cinnamon and salt and then melted butter. Crumble mixture on top of baked pie and bake for 20 minutes longer, or until apples are tender. Let cool completely and then refrigerate for at least 1 hour before serving.