1 box of frozen puff pastry, containing 2 rolls of pastry, thawed according to package directions, but still cold
1 egg, lightly beaten
Sugar for sprinkling
Preheat oven to 400°.Line a baking sheet with parchment paper.
In a bowl, combine blueberries, sugar, zest, lemon juice, cinnamon, salt and cornstarch.Stir to make sure mixture is well combined and berries are evenly coated.
Take one roll of pastry out, and place or return the second roll to the fridge to keep cold.Note that the dough should be defrosted, but still cold to the touch.If your dough is not cold, wrap it well and return it to the fridge until it is chilled.
Working quickly, unroll the pastry and cut out 8 circles using a 3” round cookie cutter or biscuit cutter. Remove the extra pastry around the circles and return to the fridge for another use, or discard. Lift 4 of the circles onto a lightly floured work surface and heap 3 Tbsp of the blueberry mixture into the centre of each of these 4 circles. Using a pastry brush, brush some of the beaten egg around the edges of the pastry, going in a circle around the blueberry mounds.The egg will act as glue to seal the pastry circles together. Pick upup one of the remaining 4 circles of pastry, and gently stretch it out with your fingers to make it a little larger. Drape the stretched pastry circle over one of the blueberry topped pastry circles. Using your fingers, press down around the blueberry mound to enclose the blueberries and seal the two layers of pastry together.Using the other pastry circles, repeat the the stretching, draping and sealing to enclose the next three pies. Transfer the 4 pies to the prepared baking sheet and place in the fridge to keep cold.
Remove the second sheet of puff pastry and repeat step 4, to make 4 more pies.Place these pies on the baking sheet and return the whole thing to the fridge as well to chill for 10 minutes. Reserve the rest of the beaten egg.
Remove the pies from the fridge. Using the tines of a fork, go around the circle of each pie, pressing down with the fork, to crimp the edges. Using the tip of a sharp paring knife, cut vent holes into the top of each pie. Brush the pies with some of the remaining beaten egg, and sprinkle each liberally with sugar.
Transfer the tray to the oven and bake for 20 minutes, until the pastry has risen and is golden brown and the berries have bubbled out of the vent holes.
Allow to cool on the baking sheet.These are best eaten the day they are made.