Heat the oven to its highest setting, or 240C fan. Put the first nine ingredients in a large bowl with the chopped sage and a teaspoon of salt and plenty of pepper. Gently mix everything together, taking care not to break the potato slices.
Transfer to a 25cm x 35cm baking dish and smooth out the surface. Pour over the stock, cover the dish tightly with foil and bake for 35 minutes. Remove the foil, turn down the oven to 200C (180C fan)/390F/gas 6, and bake for 40 minutes more, until the top is crisp and bubbling.
Turn on the grill. Mix the whole sage leaves with the oil, arrange on top of the gratin, then grill for three to four minutes, until the sage is crisp but still bright green. Leave to cool for 10 minutes before serving directly from the baking dish.
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