Method
Heat the oven to 190°C/170°C fan/375°F/gas mark 5.
Cut the potatoes into 3–4mm (1⁄8–1⁄6in) slices, either using a mandoline or by hand. If you’re slicing by hand, cut away one slice, then rest the cut surface down on the board, so that the potato doesn’t roll around so much, and slice the remainder. Rinse all the potato slices in a big bowl of fresh, cold water and drain.
Tumble the potatoes, onions, garlic, olive oil, thyme leaves and chives together with the salt and a really kind-hearted quantity of black pepper.
Arrange the potatoes over the base of a deep roasting tin or baking dish, making sure you distribute the onion relatively evenly and layering the potatoes until you’ve used them all. Pour the stock into the dish – I’d suggest you pour it into a little corner of the dish, so as not to wash away all the herbs and seasoning from the tops of the potato slices and give it a shake and a shuffle to ensure it reaches the places it’s supposed to.
Cover the tray loosely with baking parchment or foil, then bake for 1 hour 20 minutes. Uncover the potatoes and use the back of a spatula to press them down into the stock so they are nice and level. Return the potatoes, uncovered, to the oven for a final 25–30 minutes, until the tops turn nice and golden and the stock has reduced away to nothing but flavour. Allow to rest for 10–15 minutes before serving.