Place the butter in a frying pan over a medium heat and add the shallots. Turn the heat down, then add the leaves from 4 sprigs of thyme. Let the shallots cook down slowly, around 5-6 minutes, without taking on too much colour. Stir in the balsamic and let it reduce a little for the final minute. Meanwhile, peel the potatoes, then slice very thinly, ideally with a mandoline.

Bring a large pan of water to the boil, then add the salt. Season the potatoes, add to the pan, bring it back to a simmer and cook for 3-5 minutes, or until just tender, but not falling apart. Drain in a colander, then place the potatoes back in the saucepan and run under cold water. Drain again, then return to the pan.

Mix together the duck fat, garlic and 8 sage leaves, then pour over the potatoes. Gently coat them with the fat using your hands and season with salt and pepper. Preheat the oven to 190ºC, gas mark 5.

Arrange the potatoes in a large, ovenproof frying pan. Collect clusters of potato slices and stack tightly on their side, starting around the outside of the pan, then working into the middle. Between each cluster add 1 tsp shallots, the leaves from a few sprigs of thyme and a few sage leaves.

Rub 5 small bundles of thyme and a handful more sage leaves with some excess duck fat. Space the thyme bundles, each with a couple of sage leaves, over the top of the potatoes. Pour over any excess duck fat.

Add and extra sprinkle of salt and pepper, then bake for 45-50 minutes, until golden brown and crisp on top and tender in the middle. Discard the bundles of thyme, then serve immediately.