Combine flour, sugar, and salt. Cut in cold butter until mixture resembles coarse crumbs with visible pea-sized butter pieces.
Add vinegar to ice water. Drizzle over flour mixture one tablespoon at a time, tossing with a fork, until dough just comes together when pressed. It should look shaggy, not smooth.
Press into a flat disc. Wrap and refrigerate at least 30 minutes.
Toss berries with sugar, cornstarch, lemon juice, lemon zest, vanilla, salt, and cinnamon. Set aside.
Preheat oven to 200°C (400°F). Line a baking sheet with parchment.
Roll dough on a lightly floured sheet of parchment into a rough 30–33 cm circle, 3–4 mm thick. Slide onto the baking sheet.
Sprinkle almond flour or breadcrumbs over the center, leaving a 6–7 cm border clear.
Spoon berry filling over the almond flour in an even layer. Keep the border clear.
Fold the dough border up and over the edge of the filling, pleating every 5–7 cm as you work around. Press lightly to hold.
Brush the folded border generously with egg wash. Sprinkle with coarse sugar.
Refrigerate the assembled galette for 10–15 minutes before baking.
Bake 38–42 minutes until the crust is deeply golden all over and the filling is actively bubbling at the center.
Slide onto a wire rack. Cool at least 20 minutes before slicing.
Serve warm or at room temperature with ice cream, cream, or crème fraîche. Finish with mint and lemon zest.