For the pastry, place the flour and caster sugar in a food processor. Process until combined. Add the butter and process until the mixture resembles fine crumbs. Add and water and process until the pastry just comes together. Turn onto a lightly floured surface and knead lightly until smooth. Shape into a rectangle. If it’s a warm day, wrap in greaseproof paper and refrigerate for 30 minutes until the pastry firm enough to roll out.
Place a large flat tray (large enough for the galette to sit flat) into the oven. Preheat the oven and the tray to 220°C fan forced. Roll the pastry out on a sheet of non-stick baking paper to form a 30x18cm rectangle.
Place the sugar, pistachios, butter, egg yolk, flour and vanilla in the food processor. Process until well combined and nuts finely chopped. Carefully spread the mixture over the pastry leaving a 2cm boarder around all the edge.
Cut the mango cheeks from the stones. Using a large spoon, remove the mango fruit from the cheeks and thickly slice the fruit. Arrange the mango over the pistachio mixture. Fold the pastry over the mango, leaving half -two thirds of the mango exposed. Brush the top of the pastry with water and sprinkle over the raw sugar. Place the galette into the fridge to firm while preheating the oven.
Slide the galette onto the hot tray, still on the baking paper. Reduce the oven to 200°C fan forced and bake for 40-50 minutes or until the pastry is golden and crisp. Set aside for 10 minutes to cool slightly. Serve warm or cold with ice cream or thick cream.