Add flour, sugar, salt, and butter to a food processor and pulse until the butter is pea-sized. Continue pulsing while drizzling in the cold water 1 tablespoon at a time. Stop adding water as soon as the pie crust forms a ball.

Place the ball on a floured surface and work the ball into a disk. Wrap in plastic wrap and chill in the refrigerator for at least 2 hours.

Combine blackberries, raspberries, corn starch, granulated sugar, and lemon juice in a medium bowl. Gently stir until everything is combined.

Preheat the oven to 400 degrees and set aside a large baking sheet.

Roll out the chilled pie crust on a large piece of parchment paper using a rolling pin. Use your hands to patch up any cracks. The pie crust should be a large rough circle and ½ inch in thickness when done.

Scoop the berry mixture into the center of the pie crust.

Gently fold the sides of the pie crust into the center. It is ok if this doesn’t look perfect.

Once all sides are folded in, brush the edges with melted vegan butter and sprinkle with brown sugar.

Lift the parchment paper that holds the galette and place it on the baking sheet.

Bake in the oven for 35 minutes. The crust should be golden brown.

Let the galette cool completely either on a wire rack or on the baking sheet.