Mix the blackberries, blueberries, strawberries, raspberries, brown sugar, lemon juice, and corn starch in a medium-large bowl until well combined.
Assembling the Galette
Place the pie crust flat on a baking sheet with parchment paper.Note: If using a frozen store-bought crust, follow the package's instructions to thaw it before unrolling it onto the baking sheet.
Spoon the berry filling onto the center of the dough (try to leave the juice behind in the bowl). Leave about 1½ inches of the dough unfilled around the edge.
Tightly fold the unfilled dough edge over the filling. The galette should be about 8 inches in diameter.
Place the baking sheet with the galette in the freezer and preheat your oven to 375°F.
When your oven is pre-heated and the galette crust is cold and firm, brush the crust with an egg wash (the yolk mixed with water) and sprinkle turbinado/demerara sugar over the egg wash.
Bake on the middle shelf until the crust is golden and the filling is bubbling (about 25-35 minutes).
Let cool for 10-20 minutes before serving. Top with ice cream, chocolate sauce, whipped cream, or your favorite toppings and enjoy!
Notes
Fruit: If using frozen fruit, do not thaw ahead of time.Storage: Store cooled leftovers in an airtight container in the refrigerator for up to 1 week or the freezer for 1 month.