Prepare the Dough: In a food processor, mix all-purpose flour, salt, and sugar until combined. Add cold, cubed butter and pulse until the mixture resembles coarse crumbs. Slowly add ice cold water until the dough starts to form. Wrap it in cling film and chill for at least 30 minutes.

Preheat and Prepare Baking Tray: Preheat the oven to 200°C (390°F), or 180°C (365°F) for fan ovens. Line a baking tray with parchment paper.

Make the Filling: In a mixing bowl, combine blueberries, cornflour, sugar, and lemon juice. Set aside.

Roll Out the Dough: Roll out the chilled dough into a circular shape about 4mm thick.

Assemble the Galette: Lift the rolled dough onto the baking tray. Spoon the blueberry mixture into the center, leaving a 2-3 inch border. Fold the edges over the filling.

Finish and Bake: Brush edges of the dough with an egg wash and sprinkle sugar over the top. Bake for 25-30 minutes until golden brown.