Preheat oven to 400 degrees. Line 2 baking sheets with parchment.
In a medium mixing bowl, combine blueberries, 1/4 cup sugar, cornstarch, and lemon juice. Set aside.
Sprinkle clean work surface with a small amount of flour and spread thinly. Roll out one of the pie crusts from it's tubular shape. Using a rolling pin roll dough to double it's size.
Using a 4" in diameter bowl or cup, place on the dough and cut around the dish to form a 4" circle. Cut 3 more rounds. Roll out remaining dough scraps to form an additional 1 or 2 circles.
Spoon 1 1/2 - 2 TBS of the blueberry mixture into the center of each round. Fold dough edges up and around the filling without covering the center to form a rustic galette.
Place the galettes onto the prepared sheet pans. Brush each galette with egg mixture (dough only) and sprinkle sugar on the top.
Prepare final pie crust using same steps as above.
Bake in preheated oven for 25-30 minutes until golden brown.