Slice potatoes very thinly (best to use a mandolin) and place in a large bowl.
Pour over butter and thyme and season with salt and pepper. Toss well to coat the potatoes with the butter.
Arrange the potatoes in a 20cm cast iron pan, ensuring the first layer is placed attractively as this will be the top layer of the galette. It is recommended to line the pan with baking paper to ensure ease of removal of the galette.
Bake for 1 hour or until the potatoes are tender.
Allow to rest for 10 minutes before turning out onto a baking tray. Turn the oven onto the grill function.