Prep the Oven and Pan: Preheat your oven to 375°F and line a large baking sheet with parchment paper.
Whisk the Dry Ingredients: In a large mixing bowl, whisk together the ¼ cup sugar, lemon zest, and 2 tablespoons of flour.
Fold and Rest the Berries: Gently fold in the fresh blueberries and lemon juice (or lemonade). Mix well to coat the berries and let stand for 10 minutes.
Roll the Pastry: Meanwhile, on a lightly floured surface, roll out the puff pastry sheet with a rolling pin into a 13-inch round, about ⅛-inch thick. Gently transfer the pastry to the center of the prepared baking sheet.
Fill and Fold: Spoon the blueberry mixture into the center of the pastry, leaving a 1- to 2-inch border around the edges. Fold the pastry border up and over the blueberries.
Wash and Sugar: Brush the beaten egg white along the folded pastry border, then sprinkle with a pinch or two of extra sugar.
Bake: Bake on the middle rack of the oven for 40 to 45 minutes, until the pastry is a beautiful golden brown and the blueberry filling starts to bubble.
Cool and Serve: Remove from the oven and let the baking sheet cool on the stovetop. Zest some fresh lemon over the top for presentation, cut into wedges, and serve warm or at room temperature with a scoop of vanilla bean gelato or ice cream.