Add the flour, butter, salt and mixed herbs into the smaller bowl of your food processor and whiz for a minute until the mixture resembles breadcrumbs. Then add in 2 tbsp of ice cold water and process again until the dough forms a ball. You might need to add another tbsp of water. Form dough into a disk shape, wrap it in cling film and refrigerate for 1 hr.

Preheat the oven to fan assisted 160C / 180C / 350F / gas 4. Add sweet potato to one half of the baking tray and the onion to the other half. You need to keep them separate, as you will remove the onion earlier than the sweet potato. Drizzle on some olive oil and salt, mix well and roast the vegetables for about 15 mins for the onion and 20 mins for the sweet potato till they are almost done but still have some bite to them.

Preheat the oven to fan assisted 180C / 200C / 400F / gas 6. Line a baking tray with some baking parchment. Roll out the dough on a lightly floured surface into a 12 inch circle. If you want you can trim the edges. Transfer the dough on to the baking tray.

Add the sweet potato and onion mix onto the middle of the galette leaving a couple of inches clear for the folds. Drizzle on the balsamic vinegar and the sprinkle on the cheese and herbs. Fold the dough over the sweet potato mix pleating as necessary. Brush the edge of the galette with the egg.

Bake the galette for between 25 -30 mins until the crust is golden brown. Serve with a green salad with balsamic vinaigrette dressing.