Place the first 8 ingredients in a bowl. Toss to combine, and then set aside.
Line two baking sheets with parchment paper. Cut the pastry, widthwise, in half. Set one half on a lightly floured surface. Gently roll into a 10" x 10" square (does not have to perfectly formed). Set rolled pastry in centre of one of the baking sheets. Roll the second piece of dough in this fashion and set it on the second baking sheet. Brush the outer edges of each piece of pastry with egg wash.
Divide and mound the blueberry mixture on the squares of puff pastry, leaving a 2" border of clean dough. Fold the clean edges of pastry up and partially over the filling, leaving the centre open. Push and squeeze the pastry together to help hold in place. Brush the top of the pastry with egg wash. Refrigerate the galettes 30 minutes to firm up the pastry.
Preheat the oven 375 degrees F. Bake one galette 30 to 35 minutes, until puffed and golden. Set on a baking rack to cool. Repeat with the other galette. When ready to serve, dust each galette lightly with icing sugar, slice and enjoy. If desired, serve galette with whipped cream or ice cream.
Recipe Options: Make blueberry and raspberry galette by replacing half the blueberries with raspberries. Instead of puff pastry, make the galettes with pie dough. The galettes won't be as puffed, but they will still be flaky and tasty.