Combine flour, 1 tablespoon caster sugar, and fine sea salt in a large bowl. Add cold cubed butter and rub with fingertips until mixture resembles coarse breadcrumbs with some pea-sized pieces remaining.

Drizzle in ice-cold water and mix with a fork until dough just comes together. Shape into a flat disc, wrap in cling film, and refrigerate for at least 1 hour.

Preheat oven to 200°C (180°C fan). Line a large baking tray with parchment paper.

In a large bowl, gently toss rhubarb and strawberries with 100g caster sugar, cornflour, vanilla extract, and lemon zest until evenly coated.

Roll out chilled pastry on a lightly floured surface into a rough circle approximately 35cm in diameter. Transfer to the prepared baking tray.

Arrange the fruit mixture in the centre of the pastry, leaving a 5cm border. Fold the edges of the pastry up and over the fruit, pleating as you go and leaving the centre exposed.

Brush the pastry edges with beaten egg and sprinkle generously with demerara sugar and a pinch of flaky sea salt.

Bake for 40-45 minutes until the pastry is deeply golden and the fruit is bubbling and tender.

Allow the galette to cool on the tray for at least 15 minutes before slicing. Serve warm or at room temperature.