Combine flour and sugar in a food processor. Pulse to mix. Add the butter and pulse a few more times. You should still be able to see some small chunks of butter no bigger than peas.
Sprinkle 1 tablespoon of ice water over flour mixture at a time. Pulse a couple of times. If you pinch some of the crumbly dough and it holds together, it's ready. If not, keep adding water onetablespoon at a time.
Strawberry Rhubarb Filling
In a large bowl, mix cut strawberries and rhubarb with sugar and elderflower cordial. Transfer to a large strainer and place over a bowl to catch the liquid. Leave for 30 minutes at room temperature.
After the dough has chilled in the refrigerator for an hour, you can roll it out. If it is too stiff, let it sit for 5-10 minutes at room temperature before rolling.
Sprinkle a little flour on a flat, clean work surface and on top of the dough.
Place a clean baking sheet onto a work surface and sprinkle it with some flor. Using a rolling pin, roll it out from the center of the dough to about 3-4 mm (1/8 inch). Every once in a while you may need to gently lift under the dough and add a sprinkle of flour.
When the dough has reached the right size transfer the crust together with the baking paper onto a baking tray.
Place strained strawberry and rhubarb mixture over the pastry. Leave 2-3 cm edge and fold it over the fruit to form a galette.
Using a brush, cover the crust with milk and sprinkle with sugar.
Place the galette in the freezer while the oven is preheating to 200°C.
Bake the galette for 40 minutes and leave to cool before cutting.