1 tablespoon cold unsalted butter, cut into pieces
1 large egg, lightly beaten
Preheat your oven to 425°F and line a large baking sheet with parchment paper.
In a medium bowl, stir together the blueberries, cornstarch, zest, juice, cinnamon, salt, and the sugar.
Unwrap the pie dough and unfold onto baking sheet, then spoon the blueberry mixture onto the center of the dough, leaving a 1 1/2-inch border around the edge.
Fold the edges of dough over the sides of the blueberry mixture, pleating the dough lightly. Dot the blueberry filling with the butter pieces, then lightly brush the pastry with some of beaten egg and sprinkle with remaining teaspoon sugar.
Bake until the blueberry filling is bubbling and the pastry is golden, about 25 to 30 minutes. Cool slightly on baking sheet on a rack and serve warm galette with scoops of ice cream.