Summer Berry Pie

In a large bowl, stir together the flour, sugar and salt. Add the butter and cut into the dry ingredients with a pastry blender or knife until pieces are the size of peas.
Add the buttermilk and stir to make a shaggy dough. Dump the mixture out onto a clean counter and use your hands to bring it together. Add more buttermilk a tablespoon at a time, if necessary. (but it's okay if it's on the dry side).
Form into two equally sized disks. Wrap in plastic wrap and chill in the fridge for at least one hour or up to 3 days.

Preheat oven to 450°F. Line a sheet pan with parchment paper (this will catch any juices that spill over as the pie bakes).
When ready to make the pie, unwrap one of the chilled disks of dough and roll out to about ¼ inch thick on a well-floured surface. Transfer to a pie dish, trim the crust to about ½ overhang and chill until you're ready for it. Meanwhile, roll out the second disk.
Roll out the second disk and have it ready to go on your counter.
In a bowl, combine the sugar, cornstarch, instant tapioca, lemon zest, and salt. Add the berries and toss gently until covered. Pour into the prepared chilled pie crust.
Cover with the second crust and pinch the edges together tightly to seal, and crimp into a fluted pattern. Brush with the beaten egg and sprinkle with the sanding sugar. Cut 3 vents in the center of the crust to allow steam to escape.
Place pie on the prepared baking sheet and bake at 450°F for 10 minutes. Reduce oven temperature to 375°F and bake for an additional 30-40 minutes, or until crust is golden brown and juices are bubbly in the center. If the crust browns before the filling is ready, cover with foil.
Cool 3 hours, or until pie is room temperature. (This ensures pie filling has thickened properly). Slice and serve with vanilla ice cream, if desired.
Store pie covered with plastic wrap at room temperature for up to 24-48 hours max, or in the fridge for up to 5 days.