Make the pastry dough by combining flour, sugar, and salt in a large bowl. Cut in the cold butter using a pastry cutter or food processor until the mixture resembles coarse crumbs with some pea-sized pieces remaining. Gradually add ice water, 1 tablespoon at a time, mixing until the dough just holds together when pinched.
Form the dough into a disk, wrap in plastic wrap, and refrigerate for at least 30 minutes. This chilling step is crucial for flaky pastry.
Prepare the filling by gently mixing blueberries, sugar, cornstarch, lemon zest, lemon juice, and cinnamon (if using) in a bowl. Let stand for 10 minutes to allow flavors to meld.
Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
Divide the chilled dough into 6 equal portions and roll each into a rough 6-inch circle about ⅛-inch thick. Don’t worry about perfect circles – the rustic look is part of their charm!
Transfer the dough circles to the prepared baking sheet, leaving space between each. If the dough becomes too soft, refrigerate briefly before continuing.
Place about ⅓ cup of blueberry filling in the center of each dough circle, leaving a 1½-inch border around the edges. Avoid overfilling or your galettes might leak.
Fold the edges of the dough up and over the filling, pleating as you go, leaving the center exposed. The folds should overlap slightly to seal in the juices.
Brush the pastry edges with egg wash and sprinkle with additional sugar for a beautiful finish and extra crunch.
Bake for 40-45 minutes until the crust is golden brown and the filling is bubbling. Some juice may leak out – that’s perfectly normal and adds to their rustic appeal.
Allow to cool for at least 15 minutes before serving to let the filling set slightly.