Combine flour and salt. Cut the butter into the flour using either a pastry cutter or two forks, until you reach a crumbly texture.
Slowly drizzle in water until the dough can be formed into a loose ball. Divide into two equal balls, pat into disk shapes, wrap in plastic wrap, and set in the fridge to firm up for at least an hour.
Mix together your blueberries, cornstarch, sugar, lemon zest, cinnamon, and salt.
Preheat oven to 400 degrees F. Remove one of the pie crust disks from the fridge and roll into a large circle (at least 1 inch larger than your pie pan, I recommend a 9 inch pie pan). Transfer the crust into the pie pan, leaving ½ inch overhang on all sides.
Spoon blueberry filling into the crust, gently evening out the top.
Remove the other pie crust from the fridge and roll into an equally large circle. Cut dough into 1 inch wide strips.
Make your woven lattice pie crust on top. To do this, lay a row of dough strips going one direction (horizontal). Fold half of the strips back over themselves, then lay a strip going the other direction (vertical). Lay the horizontal strips over the vertical, then fold the other half of the horizontal strips to repeat, weaving the dough strips to form a lattice pie topping.
Brush dough with whisked egg, optionally sprinkling with a bit of sugar.
Set pie in the oven, with a baking sheet below it to catch drips. Bake for 20 minutes at 400 degrees F, then reduce temperature to 350 degrees F and continue baking for 35 minutes, or until filling is bubbling at the edges. If you notice crust beginning to brown too much, cover the edges with aluminum foil.
Allow pie to cool before serving to allow blueberry filling to thicken.
If you don't have a pasty cutter, you can use two butter knifes scissor style to cut the butter into the dry ingredients.