Pie dough:

1. Place all dry ingredients into a food processor and pulse to evenly distribute
2. Add the cold, cubed butter to flour mixture and pulse until the mixture resembles coarse meal.
3. Place mixture into a large bowl and set aside.
4. Combine egg, water, and buttermilk together in a small bowl.
5. Create a well in the flour mixture, gradually whisk in the egg mixture with a fork.
6. Mix the dough until it forms a ball.
7. Divide the dough in half and flatten each half into a disc, about 5-inches in diameter. Wrap each disc in plastic wrap and refrigerate for at least 1 hour and up to 3 days in advance or freeze for up to one month in advance.
8. Remove dough from the refrigerator 15 minutes before rolling. Use Cup4Cup Multipurpose Gluten Free Flour to lightly dust your work surface. Roll one disc into a 12-inch circle, about 1/8-inch thick, lifting the dough often to prevent it from sticking. For reference, 1/8-inch is about the same thickness as two pennies stacked together.
9. Fit rolled-out pie dough into a standard 9-inch pie tin. Be sure to use a pie tin that is made from thick, sturdy metal with a flat rim for the best baking.
10. Roll out the second disc of pie dough as directed above then cut it into strips about 1-inch wide. Weave the strips into a lattice on top of the apple filling. Press the bottom crust edge and lattice edge together and trim the dough so that it overhangs the pie dish by about an inch. Roll the edges of the dough underneath itself to form a rounded edge. The edge of the dough should remain on top of the flat surface of the pie dish. Use your fingers to form a fluted edge or crimp it with a fork if you prefer. Brush the edge and lattice with the egg wash and sprinkle it with the coarse sugar.

Filling:

1. Preheat oven to 350.
2. In a large bowl, mix the blueberries with the lemon zest and juice, ginger, Cup4Cup flour, sugar, and salt. 
3. Place the fruit mixture in the dough-lined pie dish.
4. Dot the top of the filling evenly with the 2 tablespoons cubed butter.
5. Moisten the edges of the pie dough, then lay down the remaining disk of dough or weave the lattice strips on top of the filling. Press the edges to the crust together to seal then use a paring knife to trim the excess dough from the edge. Crimp the crust with your fingers or a fork. If you are preparing a full crust, cut a few slits slit in the top to allow steam to escape.
6. Whisk the egg with the water to make an egg wash.
7. Brush the top of the dough with the egg wash and sprinkle the coarse sugar on top.
8. Bake for 45 minutes to 1 hour or until golden brown and the filling is bubbling.
9. Cool for at least 1-2 hours to allow the filling to set before slicing.