Add the washed blueberries, sugar, cornstarch, lemon juice and water to a sauce pan or small pot. Mix all the ingredients to ensure all the blueberries are well coated. Bring the blueberries to a simmer over medium heat. Once the blueberries reach a simmer, cook for 4 - 5 or until the blueberry sauce has thickened to a jam like consistency. There will be lumps of blueberries throughout.
1½ cups fresh or frozen blueberries, 2 tablespoons (25 g) granulated sugar, 1 tablespoon cornstarch, 1 tablespoon (15 ml) lemon juice, 1 tablespoon (15 ml) water
Transfer the filling to a heat safe bowl and cool to room temperature. Making the blueberry sauce takes about 15 - 20 minutes, with another 20 - 30 minutes to cool to room temperature. Make sure to pull the puff pastry to thaw at room temperature for about 30 - 45 minutes. Preheat the oven to 400 F (204 C). Line 2 large baking sheets with parchment paper. Unfold one sheet of puff pastry to form a square. Cut the square of puff pastry into four smaller squares. Imagine a line running diagonally from the bottom left corner of the pastry square to the top right, dividing the square into two triangles. Spoon 1 ½ - 2 tablespoons of cooled blueberry filling slightly above the (imaginary) line into the middle of the upper triangle. Repeat adding the filling into each of the remaining squares. 1 box puff pastry , ½ - ¾ cup (120 - 180 ml) blueberry filling
Lightly brush the edges of the puff pastry squares with the lightly beaten egg. Reserve about half of the lightly beaten egg to brush across the top of the blueberry turnover pastries before baking.
1 egg, lightly beaten
Lift the lower right corner up to the upper left corner. Gently press the corners together. Press the edges of the puff pastry together so that a triangle forms. It’s ok if some filling comes out. Simply wipe away any filling that comes out. Using a fork, press the edges of the puff pastry together to seal the seam.
Lightly brush the top of each cherry turnover with the lightly beaten egg. Sprinkle some coarse sugar on top if you like. If the turnovers have gotten a bit warm, transfer the baking sheet into the fridge to chill for 15 - 30 minutes. Chilling helps the puff pastry dough puff up really beautifully.
2 - 3 tablespoons (25 - 38 g) sugar for topping, optional
Bake the blueberry puff pastry turnovers for 18 - 22 minutes or until the pastries are a golden brown color. Cool on the hot baking sheet for 3 - 5 minutes. Transfer to a cooling rack to cool to room temperature.
Make the vanilla glaze by whisking powdered sugar with milk and vanilla extract until smooth the well combined. Drizzle the glaze over the cooled turnovers. Enjoy warm or at room temperature.
1 cup (120 g) powdered sugar, 2 tablespoon (30 ml) milk, ½ teaspoon vanilla extract