Preheat the oven to 375℉ and line a rimmed cookie sheet with parchment paper.
Place the puff pastry onto the pan and chill it in the refrigerator while you prepare the galette filling.
In a large bowl, stir whisk together the sugar, tapioca, cardamom, and salt. Mix in the berries.
Remove the puff pastry circle from the fridge. Pour the berry filling into the center of the puff pastry, and spread it out into a circle leaving a 1 1/2'' border around the edges. It’s okay to heap the berries slightly in the center.
Fold the edges of the pastry over the filling and gently brush the beaten egg between the folds to seal. Brush the rest of the pastry with egg wash and sprinkle the entire galette with demerera sugar.
Dot the top with small chunks of butter.
Bake until the fruit juices bubble in the center and the pastry is a deep golden brown, about 30-40 minutes.
Let cool before serving. Cerve with vanilla ice cream.