In a medium saucepan over medium heat, mix wild blueberries, granulated sugar, water, and cornstarch. Stir until the cornstarch dissolves.

Keep stirring for 5-7 minutes until the mixture thickens into a shiny gel. Remove from heat, add lemon juice, stir, and let it cool completely to avoid a soggy crust.

Thaw and unfold the puff pastry sheets on a lightly floured surface. Cut each sheet into four rectangles, making a total of eight. Gently stretch any stiff areas to prevent cracking.

Add the Egg Wash and Filling: Place four pastry rectangles on a baking sheet lined with parchment paper. Brush the edges with egg wash and add 2 to 3 tablespoons of cooled blueberry filling in the center of each.

Cover each filled rectangle with another piece of pastry. Press down firmly on the edges and use a fork to crimp them shut, making sure there are no weak spots.

Brush the tops of the pies with egg wash, and cut 2 to 3 small slits in the center to let steam escape. Sprinkle a little bit of turbinado sugar over each pie for extra crunch and color.

Bake at 400°F for 20 to 22 minutes, until golden brown and bubbling. Let them cool on a wire rack for at least 10 minutes before serving.