Combine apples, 2 tablespoons flour, nutmeg, and salt. Mix together egg, sour cream, 3/4 cup sugar, and vanilla; blend into the apple mixture. Pour filling into unbaked pie shell.
Cream together 1/3 cup sugar, 1/3 cup flour, and 1/4 cup butter or margarine. Set topping aside.
Bake for 15 minutes. Reduce temperature to 300 degrees F (150 degrees C), and continue baking for 30 minutes. Remove pie from oven, and sprinkle with topping. Increase temperature to 400 degrees F (205 degrees C), and bake pie for 10 more minutes. Cool.
