Make the galette as per the instructions. Chill for at least one hour. Preheat the oven to 400F, line a baking sheet with parchment paper
In a large bowl, mix together the blueberries, lemon zest, salt, vanilla, sugar, and flour together. Set aside.
Divide the galette dough into two balls. Flatten and roll out into circles that are ¼ inch in thickness. Place one of the galette discs onto the baking sheet. Place half the blueberry mixture onto the disc, spreading out the blueberries out in an even layer, leaving a one-inch border along the dough.
Fold the border onto the blueberries, folding the dough onto itself as necessary. Place the second disc onto the baking sheet and add the blueberries and fold as before.
Brush the egg wash onto the folded over dough and sprinkle with turbinado sugar.
Bake for 25 - 30 minutes, or until the dough is a nice golden brown and the juices of the blueberries are bubbling.
Allow the galette to cool completely before serving.