Begin by preparing the rosemary, which should be stripped from the stalks then bruised in a pestle and mortar.After that, take two-thirds of the leaves and chop them finely. Now cut the onions in half and then the halves into the thinnest slices possible; the potatoes should be sliced, but not too thinly. All you do is arrange a layer of potatoes, then onions, in the dish, followed by a scattering of rosemary, then season. Continue layering in this way, alternating the potatoes and onions and finishing with a layer of potatoes that slightly overlap.Now mix the stock and milk together and pour it over the potatoes. Season the top layer, then scatter over the whole rosemary leaves. Now put little flecks of the butter all over the potatoes and place the dish on the highest shelf of the oven for 50-60 minutes, until the top is crisp and golden and the underneath is creamy and tender.