To make the crust:
In medium bowl, combine flour, salt and sugar. Add cubed butter and use pastry blender to cut butter into dry ingredients until size of small peas. Add water 1 tablespoon at a time until dough starts to come together into ball. Dump dough out onto clean surface and knead a few times to incorporate all of dry bits. Work quickly, do not over handle.
Divide dough in half and shape into 2 disks. Cover with plastic wrap and refrigerate for at least 2 hours. Note: for this recipe you’ll only need 1 disk. Refrigerate or freeze other disk for another pie.
On a lightly floured surface, roll out the chilled pie crust into a rough 12-inch circle and transfer to a 9-inch pie plate. Trim edges and crimp as desired.
To make the filling & topping:
Preheat oven to 400 °F.
In a large bowl whisk together the sour cream, all-purpose flour, salt, eggs and vanilla extract. Add apple slices and toss to coat. Dump into the prepared pie crust.
In a second bowl combine the brown sugar, flour and cinnamon with a fork. Add the melted butter and stir to combine. Use your hands to crumble the topping on top of the pie.
Place the pie on a sheet pan and bake until topping is golden and filling is set and slightly bubbles at the edges, about 1 hour. If topping browns before filling is set, cover with foil for the remainder of baking time. Let pie cool to room temperature before serving.