Preheat oven to 180°C/350°F. On a lightly floured surface, roll out the puff pastry into a circle that is roughly 30cm (12 in) in diameter, trimming if necessary. Transfer to a parchment-lined baking sheet and refrigerate while mixing the filling.
Add the blueberries to a large mixing bowl. Add the brown sugar, flour, lemon juice and salt. Stir until well combined.
Remove the pastry from the fridge. Pile the filling in the centre of the pastry, leaving a 5cm (2in) border. Fold the border over the filling. Brush the exposed pastry with the beaten egg and sprinkle generously with demerara sugar.
Move to the oven and bake until the filling is bubbling and the pastry is deeply golden brown and crisp - 30-40 minutes. Allow the tart to cool completely before serving.
Notes
Use the weight measurement of salt over the volumetric if at all possible. The volume of different styles and brands of salt can vary drastically so going by weight will give you the most consistent results.