In a medium saucepan, combine blueberries and sugar. Cook over medium heat until blueberries begin to release their juices, 4 to 6 minutes, stirring occasionally.

Meanwhile, mix cornstarch and lemon juice together in a small bowl until combined. Stir cornstarch slurry into the blueberry mixture, then bring to a gentle boil, stirring often.

Once boiling, remove from heat. Let cool completely.

Cut puff pastry sheet into 6 rectangles. Spoon 3 tablespoons of cooled blueberry mixture onto each of the 3 bottom rectangles.

Place the top rectangles over the blueberry filling, lining up the edges as best you can. Crimp the edges with a fork to seal shut. Repeat this entire step with 2nd pastry sheet so you have a total of 6 pastry pies.

In a small bowl, whisk the egg. Brush tops of pastry pies with egg wash, then sprinkle with coarse sugar. Chill pies in the refrigerator for at least 25 minutes.