Preheat oven to 400 degrees F. Line a baking sheet with parchment paper and set aside.

In a small pot over medium heat, add the blueberries, sugar, cornstarch, water, and salt. Stir to combine and cook for 3 minutes, until bubbly and slightly thickened.

Remove from heat, add fresh lemon juice, and stir to combine. Let mixture cool while you prepare the puff pastry (or place it in the fridge for faster cooling).

Onto a lightly floured surface, unroll thawed puff pastry and lightly roll with a rolling pin to smooth out dough into a square shape.

Using a sharp knife, cut each pastry sheet into 4 equal-sized squares (you will have 8 total). Place 1-2 tablespoons of blueberry filling in center of each square.

In a small bowl, whisk together the egg and water to create an egg wash.

Use a pastry brush or spoon to brush egg wash along the ½" edge of each pastry square. Fold opposite pastry corners together to form a triangle. Use a fork or your fingers to crimp the edges together by pressing down gently.

Use the same fork to pierce the top of each pastry to allow steam to escape.

Transfer turnovers to prepared baking sheet and brush each turnover with egg wash. Bake for 15-20 minutes, until pastry is golden brown.

Allow to cool for at least 15 minutes to avoid burning your mouth. They are worth wait!

In a small bowl, whisk together the confectioner's sugar, milk, and vanilla extract until smooth. You should have a thicker consistency. Add more confectioner's sugar or milk to achieve your desired consistency.

Drizzle the glaze over turnovers and serve with any remaining blueberry filling on the side. Enjoy!