Begin by peeling and cutting the onions in half and then the halves into the thinnest slices possible; the potatoes should be sliced, but not too thinly (about 3-mm thick).Now all you do is arrange a layer of potatoes, then onions, in the dish, followed by a scattering of chopped sage, 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.Now put little flecks of butter all over the top and place the dish on the highest shelf of the oven for 1 hour, by which time the top will be crisp and golden and the underneath creamy and tender.Then leave the potatoes to get cold (don’t put them in the fridge).To reheat, cook on the middle shelf at gas mark 7, 425°F, 220°C, for 25 minutes. Dip the sage leaves in a small dish of olive oil and scatter them over the top of the potatoes 10 minutes before the end of the reheating time.