Preheat oven to 400°F / 200°C and line a baking sheet with parchment paper.
In a medium bowl, toss together the blueberries, lemon zest, lemon juice, sugar, and salt (see notes below).
Roll out the puff pastry onto the prepared baking sheet and cut it into 4 long rectangles (see pictures above).
Mound some blueberries in centre of one half of each rectangle. Fold the puff pastry over, and press the edges to seal. Brush with the egg wash, and sprinkle with raw cane sugar. Cut small slits in the tops.
Transfer the pies to the preheated oven, and bake rotating the sheet halfway through, until juices are bubbling and pastry is golden brown, 20-25 minutes (some of the juices will run onto parchment paper).
Transfer to a wire rack. Serve warm or at room temperature.
Notes
Depending on preference, you can also toss 1 1/2 teaspoons arrowroot powder or other starch with the blueberries, this will thicken up any of the juices that the blueberries release.
If you do not have any fresh berries on hand, you can substitute with frozen blueberries straight from the freezer. Frozen berries have a higher moisture content should definitely be tossed with arrowroot powder or other starch (I recommend 1 1/2 tablespoons) since they will release a lot of their juices.
For reheating the hand pies, warm them up in the oven at 325°F/165°C for about 4-5 minutes just before serving.