Preheat the oven to 200°C, and line a baking tray with parchment paper.
In a medium bowl, gently combine the blueberries, lemon zest + juice, and sugar. Leave to macerate while you ready the pastry.
On a lightly floured surface, roll the puff pastry sheet out to a large square/rectangle about 4mm thick. If using homemade puff pastry, chill in the fridge for 15-minutes before cutting.
Using a 3.5-inch heart shaped cookie cutter, cut out 24 little hearts; discard any scraps. Place the hearts on the prepared tray.
Draining as much liquid out as possible, spoon small mounds of blueberries into the centre of 12 of the hearts, leaving a 1/4-inch boarder around the edges. Brush this boarder with the egg-wash.
Gently score the top of the 12 hearts without blueberries, and place these on top of the other six. Gently press down onto the egg-washed boarder. Use a fork to crimp around the edges and seal the pies. Chill in the fridge for 10-minutes.
Lightly brush the chilled hearts with egg-wash and sprinkle generously with demerara sugar. Bake in the preheated oven for 20-minutes or until puffed and golden brown all over.