Preheat oven to 425 degrees
Place 1/2 cup sugar, 2 Tablespoons flour and 1/8 Teaspoon of salt in a medium bowl,stir to combine. Add the sour cream, egg and vanilla to the flour mixture, whisk until smooth. Add the chopped apples, fold to incorporate them into the filling.
Pour the filling into a homemade or premade store bought pie crust. (If using a premade crust follow package directions for thawing before filling the pie crust.)
Bake at 425 degrees for 15 minutes, reduce the temperature to 350 degrees and bake for an additional 30 minutes.
After the pie has baked for 30 minutes, remove it from the oven and sprinkle the crumb topping over the pie evenly. Place the pie back into the oven and bake an additional 15 minutes or until the topping is light brown and apples are tender. Remove from the oven, cool on a wire rack. Refrigerate pie at least 1 hour to chill before serving. Store leftovers covered in the refrigerator.