In a medium saucepan, combine the lemon juice, lemon zest, and granulated sugar over medium heat. Whisk in one egg yolk until fully incorporated. Stir for 5-7 minutes until it thickens and coats the back of a spoon. Remove from heat and cool to room temperature before refrigerating.

Preheat your oven to 400°F (200°C). Roll out the puff pastry on a lightly floured surface to about 1/8 inch thick. Cut into squares or circles and place on a baking sheet lined with parchment paper. Prick centers with a fork to prevent excessive puffing. Bake for 15-20 minutes until golden brown and puffy. Let cool on a wire rack.

Once cooled, gently press down the centers with fingers or a small spoon to create a well for the lemon pastry cream.

Fill a piping bag with the chilled lemon pastry cream and fill each cooled pastry shell. Top with fresh blueberries.

Brush tops lightly with milk or heavy cream. Sprinkle granulated sugar on top for crunch.

Return pastries to the oven and bake for an additional 5-8 minutes until sugar is caramelized. Remove and let cool slightly before serving.