1. Take the butter out of the fridge and cut it into cubes. Leave the butter out of the fridge to soften for 10 minutes.
2. In a mixing bowl add the flour and salt. Add the butter to the flour and using your fingers, smush the butter and flour together until it resembles breadcrumbs. Add in the chopped thyme leaves
3. Add the sour cream to the mixture, mixing everything together with a spoon initially, and then your hands, until the mixture is well incorporated.
4. Shape the pastry into a ball using your hands. Press the ball into a flat disk, wrap the disk in baking paper and place the dough in the fridge for an hour to rest. This can be done in advance
5. Place a suitably-sized pot (with a lid) on medium heat and add the olive oil and onions. Place the lid on and cook the onions gently for 20mins, stirring occasionally to stop the base from burning. Once cooked, add the sage and salt to taste and allow to cool as hot onions will lead to soggy pastry.
6. Peel and slice the butternut into thin 2-3mm thick rounds. Set aside.
7. Heat the oven to 180C.
8. To roll out the pastry, scatter flour over a work surface, place the pastry on top and roll out the pastry into a circle roughly 40cm across.
9. Roll the pastry around the rolling pin and transfer it onto the baking sheet lined with baking paper. Spoon the onions into the middle of the pastry and spread them out into a circle, being sure to leave a 5cm border of pastry to fold in to form the galette.
10. Place the butternut disks on top of the onions to form an overlapping ring (see image).
11. Slice the crotin cheese into thin disks and tuck these between a few of the butternut rounds. Sprinkle over a few pinches of salt and some freshly ground black pepper. Scatter over the crotin goats cheese.
12. Fold in the border of pastry to partially encase the butternut filling.
13. Using a pastry brush, brush the pastry with the beaten egg. Pop the galette into the oven and bake for 30-40 minutes or until the pastry is golden, and the butternut is cooked.

Serve warm, with a simple green salad.