In a large bowl, whisk together the flour, sugar and salt until combined. Cut cold butter into the flour using a pastry cutter, two knives, or a food processor, until the mixture resembles coarse meal.
If using a food processor, transfer mixture back to the bowl. Add ice-cold water, one tablespoon at a time, and using your hands, mix together until dough forms.
Divide the dough mixture into two even sized mounds. Use your hands to form each one into a disk and sprinkle lightly with flour. Wrap each disk tightly in plastic wrap and refrigerate for at least two hours.
Preheat the oven to 400 degrees F.
In a large bowl, mix together frozen Wild Blueberries, sugar, lemon juice, cinnamon and tapioca starch. Let stand for 15 minutes.
Meanwhile, working with one disc at a time, roll out dough onto a lightly floured surface, about 12 inches in diameter, 1/8 inch thick.
Carefully place the first crust onto a 9-inch pie dish and gently press the dough down against the bottom and sides of the dish.
Pour the berry filling into the pie shell. Scatter pieces of butter over top.
Place the second crust over the pie, crimp the edges to seal and make a few slits in the top crust. Or, you can cut the second crust into strips and weave to make a lattice top.
Brush the top crust with the egg wash. Bake for 45-50 minutes until golden-brown and juices are bubbling.
Transfer pie to a cooling rack and bring to room temperature before serving.
If you don’t have tapioca starch on hand, you can substitute ¼ cup flour but the juices will run slightly.