Preheat oven to 425 degrees. Prepare two rimmed baking sheets with parchment paper. In a small bowl, combine ¼ teaspoon cinnamon with 1 tablespoon sugar; set aside.
Thaw refrigerated pie dough according to package instructions. This recipe uses both dough circles typically in one package. Set each one on top of a baking sheet. Set aside.
In a large bowl, stir together blueberries, cornstarch, lemon zest and juice, and remaining 1 tsp cinnamon and ½ cup sugar until blueberries are well coated.
Spoon half of the blueberry mixture onto the center of one dough; repeat with the other half onto the other dough.
Gently fold dough over part of the blueberry mixture, and then continue around the circle to create pleats. Place half of the butter cubes on top of one blueberry center, and then repeat with the remaining half on the other one.
Use a pastry brush to lightly glaze the tops of each dough with the egg wash. Sprinkle the prepared cinnamon sugar mixture evenly over both of the galettes.
Bake at 425 degrees for about 20-25 minutes, just until the tops of the dough become lightly golden. Cool on a baking rack for a few minutes before serving.
Notes
One comment I hear a lot is about berries overflowing and bursting through/over the galette crust.Personally, I don't mind this if it happens a bit over an edge or so. But if you're a color-within-the-lines kind of gal/guy and don't want your berries to do this while baking, I suggest two things.
Dry off your berries well with a paper towel after washing them before placing them onto the crust. This will help remove excess water that can cause more liquid overflow.
Don't overfill. You should place enough berries on your dough center to cover the dough, but don't pile a huge mound of berries too high that they'll collapse and bubble over the galette edges when baking.
Extra tip: For an ever sweeter treat, serve the galette with a scoop of your favorite ice cream!