Combine Gluten-Free All-Purpose Flour (2 cup), Xanthan Gum (3/4 teaspoon), Baking Powder (1/4 teaspoon) and Salt (1/2 teaspoon) in a large mixing bowl.
Grate the Butter (6 tablespoon) onto a plate and then empty into your dry ingredients.
Work the butter in between fingers pressing it into the flour mixture to form a shaggy dough.
Add Sour Cream (1/2 cup) and, working on a floured surface, knead the dough until it holds together. Keep additional Water (1/4 cup) on hand in case you need more to loosen the dough. Form the dough into a disk and chill for 30 minutes.
Divide the dough in half. Using a floured rolling pin on a floured work surface, roll out the dough until it reaches ½-inch all around. Shape into the pie plate and chill in the fridge for another 30 minutes.
When the time is almost up, preheat oven to 375 degrees F (190 degrees C).
Using a fork, poke holes in the bottom of your crust. Careful not to pierce all the way through the dough. Prebake the crust for 10 minutes with pie weights before using as desired. Once done, place the pie crust in the fridge to cool.
Preheat oven to 350 degrees F (175 degrees C).
While the crust cools, zest and juice one Lemon (1), then set half the zest aside. Macerate Fresh Blueberries (3 cup) in Granulated Sugar (1/2 cup), Corn Starch (2 tablespoon), lemon juice and half the lemon zest, and let sit for 15 minutes.
Pour the berry mixture into the cooled pie crust. Roll out the other half of the Pie Crust and roll into a 1/2-inch thick circle the same diameter of the pie.
Place the dough on top of the pie and using a paring knife, cut slits in the center. Brush the top of the pie with Egg (1). Bake for 30 minutes, until the crust is golden brown and the filling is bubbling.
Allow pie to cool slightly before sprinkling the reserved lemon zest over the top and serving! Taste great alone or with ice cream or whipped cream.