Preheat the oven to 400F. Roll the pie crust out and place in a pie dish. Generously prick the sides and bottom, then place a piece of parchment paper or foil over the crust. Place pie weights over the paper and par-bake the crust in the preheated oven for 10 minutes. Remove from the oven and remove the weights and the parchment paper.

Meanwhile, stir together 2 tablespoons flour, salt, 3/4 cup sugar and nutmeg in bowl. Combine egg, sour cream and vanilla in another bowl; mix well. Add egg mixture to dry ingredients; mix well. Stir in apples and spoon mixture into the pie shell.

Bake the pie for 15 minutes.

Reduce temperature to 350F and bake 30 minutes more. (If the sides of the crust are getting too brown, cover them with foil.) Remove pie from oven. Increase temperature to 400F.

Combine 2/3 cup sugar, 2/3 cup flour and 2 teaspoons ground cinnamon in bowl. Cut in 4 tablespoons butter until crumbly, using a pastry blender or a fork.

Sprinkle crumb topping over pie. Return to oven and bake 10 minutes more. Cool on rack.