Adjust the oven rack to the lower third position and preheat the oven to 400°F. Line two large baking sheets with parchment paper or a silpat mat. Remove the puff pastry from the freezer if you haven't already.
Combine the blueberry filling ingredients in a medium saucepan over medium heat. Bring to a gentle simmer then turn the heat down to medium low and continue to cook, stirring frequently, for 3-5 minutes, until the blueberries burst and the sauce thickens to a jam consistency.
1 1/2 cups blueberries, 1/4 cup granulated sugar, 1 Tablespoon cornstarch, 2 Tablespoons water, Zest and juice from 1/2 lemon
Place the sheets of puff pastry on the prepared baking sheets and cut them down the middle into 4 squares each (8 total).
1 package (2 sheets) frozen puff pastry
Spoon 1-2 Tablespoons of the blueberry filling into each puff pastry square. Be careful not to overfill.
Fold one corner over to meet the opposite so that it forms a triangle. Pinch the edges closed with your fingers or press with the back of a fork. Make a small slit in the top of the turnover to let the steam escape. If you have time, freeze the filled pastries for 10-15 minutes for the flakiest crust.
Whisk the egg in a small bowl then brush the tops of the blueberry turnovers with the egg wash. Bake in the preheated oven for 15-20, until pastry puffs and the tops turn golden brown. Allow to cool for 5 minutes on the baking sheet then transfer to a wire rack to cool completely.
1 large egg
Whisk the powdered sugar and milk in a small bowl. Place parchment paper under the baked turnovers and drizzle the icing on top let it set and enjoy! Turnovers stay fresh at from temperature for up to 2 days or in the fridge for up to 5. Frozen turnovers are good for up to 3 months. Thaw overnight in the fridge.
1 cup powdered sugar, 2 Tablespoons milk