In a food processor add 2 1/2 cups of all-purpose flour, 2 tablespoons granulated sugar, 1/2 tsp of salt, and 1 1/4 cup of cold unsalted butter. Pulse on high speed until butter becomes the size of a pea.
Place the dough ingredients into a large bowl and pour 1/2 cup of buttermilk in. Then mix it all together.
Form the don’t into a bowl and divide it into two equal pieces. Then shape each piece of dough into a disk and cover with plastic wrap. Refrigerate for about 30 minutes to 1 hour.
While the dough is in the refrigerator begin making the blueberry pie filling. In a large bowl add 5 cups of blueberries, 1 1/2 tsp cinnamon, 1/2 tsp salt, 1/2 cup brown sugar, 1/4 cup granulated sugar, and 4 tbsp cornstarch. Roughly the dry ingredients, then add 1 tsp vanilla and 1 tbsp lemon juice. Mix again.
Take out 1 of the dough pieces and place it on a floured surface. Using a rolling pin roll it out to about 13 to 14 inches in diameter.
Then place the dough into a 9-inch pie dish and place the blueberry filling inside. Then evenly place 1 tbsp of unsalted cubed butter on top.
Now roll out the second piece of dough to about 13 inches in diameter and slice out 12 (3/4 incstrips using a pastry wheel.
Place 6 strips evenly spaced on top of the pie dish vertically from you. Then pull back the odd numbers (1,3,5) now lay 1 strip of Dough across. Placed the odd strips back. Now peel back the even numbers (2,4,6) and Place another strip of dough across. Continue doing this with the odd and even numbers until all remaining strips of dough are put on.
Then trim the edges of the pie with a sharp knife.
Now brush the pie with egg wash and bake at 370° Fahrenheit for about 50 minutes.