Pastry: mix together the cornstarch, xanthan gum and flour. Add the sugar, butter, egg and vanilla and mix until combined. Dust the counter with a bit of flour and take out the mixture and start kneading and folding until well combined. Divide into two flattened balls, wrap in plastic, and chill in fridge for 20 minutes (recipe only needs 1 ball - so freeze the other one for another time).

Apple Filling: mix together the apples, sugar, cinnamon, nutmeg and salt in a large bowl. Set aside.

Roll out dough to the desired thickness, then place in a greased glass pie plate. Using a fork, poke holes in the bottom of the crust and along the sides to keep from bubbling. Set aside.

Crumble Topping: mix together brown sugar, flour and nutmeg in a small bowl. Then cut in butter until you have a course crumbly mixture.

Assemble: spoon apple mixture into the pie crust, mounding in the middle and pouring the sugary juices over top. Scatter the crumble mixture evenly over the top of the apples.

Cover the edge of the pie with tin foil and then allow to bake at 400°F for 35 minutes or until you notice that the apples are bubbly.