Mix both types of flour, powdered sugar, and salt for the dough. Add cold butter in flakes. Rub butter and flour crumbly between cold hands. Add egg, quickly bring everything together to form a dough. If the dough is too dry, add 1-2 tbsp cold water. Wrap the dough in cling film and refrigerate for at least 30 minutes.
Slice strawberries. Crush sugar and saffron in a mortar or cutter. Mix with the cornstarch. Mix with the berries. Roll out the dough on a little flour to a round about 2mm thick. Place on a baking sheet lined with parchment paper. Spread the berries on top, leaving a border of about 4cm. Whisk the egg. Fold in the edges of the dough, brush with egg, and sprinkle with sugar. Chill the galette for about 15 minutes. Preheat the oven to 200°C top/bottom heat. Bake the galette in the middle of the oven for about 30 minutes until golden brown and serve lukewarm.