Preheat the oven to 375 F, and position a rack in the lower third of the oven. Line a baking sheet with parchment paper.
Make the pie dough as instructed. When you're ready to roll out the dough and assemble your galette, roll it out on a floured surface to about 12 inches in diameter. Transfer the dough to the baking sheet. You can bake your galette directly on the parchment paper, or in a pie pan, whatever you prefer.
In a bowl, gently toss the berries with the corn starch, vanilla and all but 1 tablespoon of the sugar. Pile the berries into the pie dough.
Fold up the edges of the dough around the berries. Brush the edge of the dough lightly with the egg white, then sprinkle with the remaining 1 tbsp sugar.
Bake your galette for about 50-65 minutes, however, when baking a pie, the visual cues are much more important than an estimated time. Your crust should be golden brown, but just as important, the filling should be slowly bubbling, indicating that the corn starch has been activated to thicken the filling. If you don't see slow thick bubbles, keep baking your pie until you do. Don't be afraid of a well-baked crust, but if your crust is getting too dark for your liking, you can cover the edges with aluminum foil.
Cool your galette for about 30-60 minutes, then serve warm or at room temperature.
Notes
Store the leftover galette loosely covered at room temperature for up to 1 day, or in the refrigerator for up to 2 days.