Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper and set it aside.
In a bowl, combine the fresh blueberries, granulated sugar, lemon zest, lemon juice, and cornstarch. Gently toss the mixture until the blueberries are coated evenly. Let it sit for about 5 minutes to allow the sugar and cornstarch to dissolve slightly.
On a lightly floured surface, roll out the thawed puff pastry into a circle with a diameter of about 12 inches. Transfer the pastry to the prepared baking sheet.
Spoon the blueberry mixture onto the centre of the puff pastry, leaving a 2-inch border around the edges. Gently fold the edges of the pastry over the blueberry filling, overlapping slightly as you go. Press down lightly to secure the folds.
Brush the edges of the pastry with the beaten egg, then sprinkle the coarse sugar over the edges.
Place the galette in the preheated oven and bake for 30-35 minutes, or until the pastry is golden brown and the blueberries are bubbling.
Once baked, remove the galette from the oven and let it cool on the baking sheet for a few minutes. Serve the Blueberry Galette warm or at room temperature. Cut it into wedges and serve with a scoop of vanilla ice cream on top. Enjoy! 🙂