Preheat the oven to 400°F and line a baking sheet with parchment paper.

In a medium bowl, gently toss the blueberries with granulated sugar, cornstarch, lemon juice, and lemon zest until evenly coated.

Unfold the puff pastry on a lightly floured surface and cut it into 8 equal squares. Transfer the squares to the prepared baking sheet.

Lightly score a 1/2-inch border around each square, being careful not to cut all the way through the pastry.

Whisk the egg with the water to make an egg wash. Brush the outer borders of each pastry square with the egg wash.

Spoon the blueberry mixture into the center of each square, keeping the filling inside the scored border.

Bake for 20 to 22 minutes, until the pastry is golden brown and puffed and the filling is bubbling.

Cool the tarts on the pan for 5 minutes, then transfer to a rack to cool slightly.

In a small bowl, stir together the powdered sugar and enough lemon juice to make a smooth drizzleable glaze.

Drizzle the glaze over the slightly cooled tarts and garnish with extra lemon zest if desired. Serve warm or at room temperature.