Step 1

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.

Step 2

Add the blueberries to a large mixing bowl. Add the brown sugar, flour, lemon juice and salt. Stir until well combined.

Step 3

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.

Step 4

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.