Preheat oven to 400 degrees F.
Whisk the sour cream, sugar, egg, flour, cinnamon, nutmeg, salt, and vanilla together in a large bowl.
Add the apples to the filling ingredients and stir them together. Set aside for 30 minutes to draw out some of the apple juices.
Make and roll out the pie crust. Transfer to a 9-inch pie pan. Tuck the edges under and crimp in a decorative fashion.
Fill the pie crust with the apple filling, layering the sliced apples so they don't mound too high in the center. Slowly pour any leftover filling juices over the apples to fill in the gaps.
Bake for 15 minutes, then reduce the temperature to 350 degrees F. Bake for another 30 minutes.
Meanwhile, make the topping by combining the flour, brown sugar, cinnamon, salt, and walnuts in a bowl. Add the melted butter and stir with a fork until a crumbly streusel forms. Set in the fridge until ready to use.
Once the pie has baked for 45 minutes total, sprinkle the streusel topping on top and bake for another 20 minutes until lightly browned. Cool for at least 3 hours before slicing and serving. Refrigerate any leftovers.