In a medium bowl, mix the flour, sugar, kosher salt, and baking powder. Slice the butter into small pieces, then use a pastry blender or fork to cut it into the flour mixture until mostly incorporated and apebbly texture forms (with pea-sized or smaller pieces).
Sprinkle 4 tablespoons of the cool water over the flour, mixing gradually with a fork until the flour is mostly incorporated. Add the additional 1 to 2 tablespoons of water until all the flour is incorporated, kneading with your fingers until the dough comes together. (Resist the urge to add more water; it should come together!) Form the dough into a ball and flatten into a thick disk. Wrap it in plastic or place it in a covered container and chill the dough for 1 hour. (To make in advance, you can refrigerate the dough up to 3 days; allow to sit at room temperature 30 minutes before rolling. Or, wrap in plastic wrap or aluminum foil and freeze up to 3 months, then defrost overnight in the refrigerator before rolling.)
Preheat the oven to 400 degrees Fahrenheit.
Mix the filling ingredients in a medium bowl: blueberries, sugar, lemon zest, cornstarch, cinnamon and salt.
On a floured surface, roll the dough into an even 12” circle, leaving the edges rough (if needed, move the dough around and add a bit more flour underneath to keep it from sticking). Carefully transfer the dough to a sheet of parchment paper on a rimmed baking sheet.
Pile the blueberries in the center of the crust and spread them in an even layer, leaving at least 2 inches of dough around the outside edge. Evenly sprinkle the sugar in the bottom of the bowl over the top. Fold in the outside edges of the dough over the filling to form an approximately 2-inch crust, overlapping the folds as shown in the photos.
Whisk the egg and use a pastry brush to brush it onto the crust. Sprinkle the crust with turbinado sugar (or more granulated sugar).
Bake the galette for 28 to 32 minutes until the crust is golden brown. Remove from the oven and transfer the parchment paper to a baking rack to cool. Cool to room temperature (about 1 hour) before slicing into pieces and serving. It’s divine with a dollop of vanilla ice cream. Store leftovers for up to 4 days refrigerated; bring to room temperature before serving.