Preheat your oven to 400 degrees and line a baking sheet with parchment paper.
Roll your prepared pie crust out on a floured surface.
Cut the dough into 5 inch rounds using a cookie cutter or a small bowl as a guide. (You will need 6 rounds.)
Place the rounds on parchment paper and carefully place them in the fridge until you are ready to assemble.
In a bowl add the blueberries, sugar, cornstarch, vanilla, lemon zest and lemon juice to make the filling. Gently stir until the berries are coated.
Remove the rounds of dough from the fridge. Place 1/4 cup of filling in the center of your dough.
Carefully fold up the edges creating small rim and place on your prepared baking sheet.
Repeat with the remaining 5 rounds of pie dough.
In a small bowl whisk together the egg and milk.
Brush the edges of each galette with the egg wash and sprinkle with coarse sugar if desired.
Bake for 20-25 minutes or until golden brown.