Place the prepared pie pastry shell in the freezer for 15 minutes; preheat oven to 400°.
In a big bowl, whisk the sour cream, sugar, egg, flour, salt, and vanilla until well blended.
Add in the apples; mix well.
Turn the filling into the chilled pie shell; smooth the top with your hands, laying the apple slices down flat.
Place the pie on the center oven rack; bake for 15 minutes; decrease heat to 350°; bake for 30 minutes.
Meanwhile, make the topping: in a big bowl, combine the flour, sugars, cinnamon, salt, and walnuts; add in the butter and stir with a fork, then switch to your hands rubbing the mixture gently between your fingers until it forms uniform crumbs; refrigerate until ready to use.
Remove pie from oven and carefully dump the streusel topping in the center, spreading it evenly over the surface with your hands.
Tamp the topping down lightly.
Return pie to oven, placing it so that the part that faced the back of the oven now faces the front.
Slide a foil-line baking sheet onto the rack below to catch any spills.
Continue baking for about 20 minutes or until golden brown.
Transfer pie to wire rack to cool for at least 2 hours before serving;
You may serve this pie chilled, so after it is thoroughly cooled, cover with loosely tented foil and refrigerate at least 2 hours.