In a large mixing bowl, combine the halved cherries, 1/3 cup of brown sugar, white sugar, flour, cornstarch, lemon juice, lemon zest, almond extract, and salt. Gently toss until the cherries are evenly coated. Let the mixture sit for about 10 minutes to allow the juices to develop.
On a lightly floured surface, roll out the chilled pie dough into a 14-inch circle. Aim for a thickness of about 1/8 inch. Don’t worry if it’s not a perfect circle; the rustic look is part of the charm!
Transfer the rolled-out dough onto a baking sheet lined with parchment paper. Spoon the cherry filling into the center of the dough, leaving about a 2-inch border around the edges. Gently fold the edges of the dough over the filling, pleating it as you go to create a decorative border.
In a small bowl, whisk together the egg and water. Using a pastry brush, apply the egg wash to the exposed crust. This will give the galette a beautiful golden color when baked.
Sprinkle the remaining tablespoon of brown sugar over the crust and filling. This step adds a lovely sweetness and a bit of crunch to the finished galette.
Preheat your oven to 400°F (200°C) and bake the galette for 30-35 minutes, or until the crust is golden brown and the filling is bubbling. Keep an eye on it towards the end to prevent over-browning.
Once baked, remove the galette from the oven and let it cool on the baking sheet for about 10-15 minutes. This cooling period allows the juices to set. Serve warm or at room temperature, perhaps with a scoop of vanilla ice cream or a dollop of whipped cream.