Swedish roast leg of lamb with anchovies and potato gratin

Method

1. Heat the oven to 230°C/fan210°C/ gas 8. Weigh the lamb and calculate the cooking time, allowing 16 minutes per 450g. Using a small sharp knife, make deep narrow slits all over the leg. Cut each anchovy fillet in half lengthways, then across into three. Thickly slice the garlic cloves and cut them into matchsticks. Push 1 piece of anchovy and 1-2 matchsticks of garlic into each slit. Spread the 25g butter all over the outside of the lamb and season it well all over.
2. Lightly oil a roasting tin with a little oil from the drained anchovies, then put the rosemary sprigs in the centre with any leftover garlic. Put the lamb on top and slide it into the oven. Roast for 15 minutes, then lower the temperature to 200°C/fan180°C/gas 6 and roast for the remainder of the cooking time you’ve calculated.
3. For the potato gratin, melt 30g of the 40g butter in a pan. Add the onion and fry gently, stirring, until soft and lightly golden. Set aside.
4. Cut the potatoes into 7mm thick slices, then across into short, chunky chip-shaped matchsticks. Put the cream, milk, garlic, salt and plenty of ground black pepper into a large non-stick saucepan and bring it almost to the boil. Add the potatoes, bring back to a simmer and cook for about 10 minutes, stirring them gently now and then so as not to break them up, until they are only just tender when pierced with the tip of a small, sharp knife. Stir in the onion, nutmeg and some more seasoning to taste.
5. Grease a 2 litre shallow ovenproof dish with the remaining butter. Spoon the potato mixture into the dish and sprinkle with the grated cheese. Twenty minutes before the lamb is ready to come out of the oven to rest, put the gratin in the oven and bake for 30-35 minutes until golden and bubbling. 6. Remove the lamb from the oven, cover with foil and rest for 15 minutes. Carve and serve with the gratin and some vegetables.