This is a great side to have in your Christmas repertoire, because it’s just a case of mixing everything together and baking, giving you plenty of time to do other things.
1 hour 20 minutes
1.2kg Yukon gold potatoes (ie, about 6 medium potatoes), or another baking potato that’s somewhere in between floury and waxy, skin on and cut into 5mm-thick slices
2-3 banana shallots, peeled and cut into 5mm-thick slices
4 garlic cloves, peeled and crushed
50g rose harissa (or use normal harissa, in which case use a little less and add an extra tablespoon of olive oil)
3 anchovy fillets, drained and finely chopped (optional)
110ml double cream
80g gruyere, grated
5g parsley, finely chopped
2 lemons – zest finely grated, to get 2 tsp, and juiced, to get 2 tbsp
5g sage leaves, roughly chopped, plus 10 whole leaves to finish
Salt and pepper
400ml chicken or vegetable stock
3 tbsp olive oil
1. 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.
2. 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.
3. 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.