Preheat the oven to 220C / 425F. Leave the puff pastry out to thaw.
In a bowl, combine the blueberries, sugar, lemon zest, cinnamon, cornstarch and a sprinkle of salt. Use a spoon or your hands to mix well.
For aesthetics purposes, you can cut the puff pastry sheet into a circle. You can skip this step if you want a more rustic look.
Place the blueberry mixture in the center of the puff pastry sheet, leaving a 2-3cm/2inch border. Carefully, fold the edges towards the center and over each other.
Brush the edges with the beaten egg and sprinkle sugar over the egg-washed edges. Bake for 25-30 minutes or until puffed and golden brown. Serve with ice-cream.