Blueberry filling: In a saucepan, combine blueberries, sugar and lemon juice and simmer for 10 minutes. Stir in butter. In a cup, dilute cornstarch in 1 tablespoon of water. Add some of the blueberry sauce in and stir. Pour the cornstarch slurry into the saucepan and stir the sauce until it thickens. When ready, pour it into a bowl and set aside to cool for 30 minutes.
Blueberry turnovers: Preheat oven to 400° Fahrenheit (200° Celsius). Unroll/unfold puff pastry and roll it out if not pre-rolled. Cut into squares (I divided the pastry into 6 squares, each 4-inch/11-centimeters wide). Spoon 2 heaped teaspoons of the blueberry filling in the middle of each square.
Dip your finger in water and run it alongside the edges/sides of each square. To fold, lift one tip of the pastry and fold it over the filling towards the opposite tip to form a triangle. With your fingers, press down the edges to seal them. Double-seal the edges with a fork.
Place turnovers onto a baking tray/sheet lined with baking parchment. With the fork, pierce each turnover so the steam can escape. Brush with egg wash and sprinkle with brown sugar.
Bake in a preheated oven at 400° Fahrenheit (200° Celsius) for 15 minutes or until puffed and golden brown on top as well as on the bottom of each pastry.
One batch of my homemade blueberry sauce recipe is enough for about 8 turnovers (when using 4-inch/11-centimeter pastry squares).
Either brown or white (granulated or fine) sugar can be used.
I used about ½ pound (225 grams) of pastry dough to make 6 turnovers.
To speed up the whole process, you can make blueberry filling in advance and use it when ready to bake the turnovers. (when stored properly, it should last 3 days in the fridge).
More tips and useful information can be found in the post above this recipe card.