Preheat your oven to 375°F.
If you're using a pre-made crust, place it in a 9-inch pie dish. If you're making your own crust, roll it out and place it in the pie dish, crimping the edges as desired.
In a large mixing bowl, combine the sliced apples, sour cream, granulated sugar, flour, vanilla extract, cinnamon, and salt. Mix well until the apples are evenly coated.
Pour the apple mixture into the prepared pie crust, spreading it out evenly.
In a separate bowl, combine the flour, granulated sugar, brown sugar, and cinnamon. Add the cold diced butter, and using your fingers or a pastry cutter, mix until the mixture resembles coarse crumbs. Stir in the chopped walnuts.
Sprinkle the streusel topping evenly over the apple filling, covering the entire surface.
Bake for about 50-60 minutes, or until the crust is golden brown, and the apples are tender. Tent with aluminum foil if edges of the crust are browning too quickly.
Allow the pie to cool to room temperature before serving to allow the filling to set. Serve with a dollop of whipped cream or a scoop of vanilla ice cream if desired.