To a food processor, add plain flour, salt, sugar and pulse for about 5 seconds just until it is combined.
Add cubed cold butter and pulse for about 15 seconds just until about the flour mixture looks like crumbs in appearance. Don’t worry if you can still see some clumps of butter, it means the end result of your pastry is promising.
Add about 4 to 5 tablespoons of ice-cold water to the flour passing it through the lid while you continue to run food processor one at a time until a dough is formed. The whole process took about 20 seconds.
Place the dough on a cling film and wrap it then press it down to form a disc. Refrigerate for at least 30 minutes to an hour.
Preheat the oven to 200C/390F standard oven, 180C/365 fan oven. Prepare the baking tray by lining it with a baking paper.
Make the filling: to a bowl, add the blueberries, cornflour, plain flour, sugar, lemon juice and mix to combine and set aside while you roll the dough. (You can keep it in the fridge if you want)
Lightly flour a work surface, unwrap the dough and place on the floured bit, sprinkle a bit more flour on top of the dough. Roll out the dough into a circle if you can at around 4 mm thick.
Lift the pastry gently off the work surface and wrap it around the rolling pin. Unroll it over the prepared baking tray.
Add the blueberry filling in the middle of the dough, leaving about 2 to 3 inches out the edges, fold the edge around the filling, it is ok if it overlaps, just pleat it. Make a quick egg wash, break an egg into a bowl and whisk to combine. Brush the egg on the dough.
Finally, sprinkle about ½ tablespoon or 1 tablespoon of sugar on the dough and bake the galette in the oven for 25 to 30 minutes or until golden brown, fruit completely cooked but not dried out.