Preheat the oven to 200°C (400°F) and line a baking tray with parchment paper.
In a bowl, mix 200 grams of fresh blueberries with 2 tablespoons of sugar, 1 tablespoon of cornstarch, and 1 teaspoon of lemon juice. Set aside.
Roll out 500 grams of puff pastry on a lightly floured surface to a thickness of about 3-4 mm. Cut the pastry into 8 equal squares.
Place a spoonful of the blueberry mixture in the center of each square. Fold the pastry diagonally to form a triangle, enclosing the filling. Press the edges together with a fork to seal.
In a small bowl, whisk together 1 egg and 1 tablespoon of water to make an egg wash. Brush the top of each pastry with the egg wash.
Sprinkle the pastries with a little sugar and bake for 20-25 minutes, or until the pastry is golden brown and puffed up.
Remove from the oven and let the pastries cool on a wire rack for a few minutes before serving.