Pulse flours, salt, and xanthan gum in food processor. Pulse in cold butter. Pulse in egg. Pulse until just combined.
Wrap in plastic wrap and freeze for an hour. Roll our in between two layers of parchment paper until it is just larger than your 9” pie dish. Freeze again, then remove and press into pie dish. Bake at 350 for 10 minutes.
For the Filling
Whisk all ingredients in bowl.
Pour into par-baked pie crust. Bake at 325 for 30-40 minutes. Edges should be starting to turn brown, while inside should slightly jiggle.