Prepare the Filling: Gently toss strawberries, blueberries, raspberries, and blackberries with sugar, cornstarch, lemon zest, lemon juice, and vanilla (if using). Let sit 5 minutes.
Preheat Oven: Preheat to 400°F (200°C). Line a baking sheet with parchment paper.
Roll Out Dough: On a lightly floured surface or pastry mat, roll dough into a 12-inch circle, about ⅛-inch thick. Transfer to baking sheet.
Assemble Galette: Spoon berries into center, leaving a 1½ – 2-inch border. Fold dough edges over filling, pleating as needed.
Apply Egg Wash: Whisk egg and milk; brush over crust edges. Sprinkle with coarse sugar.
Bake: Bake 30–40 minutes, until crust is golden and filling bubbles. Check at 30 minutes. Cool 10–15 minutes.
Serve: Slice and serve warm or at room temperature with ice cream, whipped cream, or mint.
Video
Notes
Blackberries: If tart, increase sugar to ⅓ cup. Halve large berries.
Cornstarch: Use 2 ½ tsp for juicy berries to thicken filling.
Storage: Refrigerate leftovers for up to 3 days. Reheat at 350°F for 5–10 minutes.