Salted rosemary roast lamb with fennel and garlic boulangère potatoes

Method

1. Preheat the oven to 230°C/ fan 210°C/gas 8. Calculate the roasting time (a 2.5kg leg, cooked medium, should take just under 1¾ hours). Using a small, sharp knife, make small 2.5cm-deep slits all over the leg, 5cm apart. Put the rosemary leaves, sea salt, garlic and black pepper into a mortar and pestle, and crush together until it’s a rough green salt. Rub the rosemary salt all over the lamb, pushing plenty into the slits. Set aside.
2. Make the boulangère potatoes. Halve and very thinly slice the onion. Halve the fennel bulb and cut out the hard core. Lay flat-side down and very thinly slice. Peel and very thinly slice the potatoes. To stop them going brown, cover with damp kitchen paper but don’t put in water, or they’ll curl up like crisps. Butter a 20cm x 30cm shallow, ovenproof dish. Layer up the potatoes, garlic, onion and fennel, finishing with the potatoes. Dot with the butter and pour over the stock.
3. Put the lamb, rounded side up, in a roasting tin, drizzle with oil and roast in the oven for 15 minutes. Lower the oven temperature to 200°C/fan180°C/ gas 6. Put the potatoes on a lower shelf and cook for 1½ hours. Continue to roast the lamb for the rest of the calculated cooking time. Check that the potatoes are not browning too much – cover with foil if they are.
4. Turn the oven off and leave the potatoes inside while you remove the lamb to a board. Cover with foil and rest for 15-20 minutes before carving.
5. Meanwhile, make the gravy. Pour away the excess fat from the roasting tin and place over a medium heat. Sprinkle with the flour and stir into the meat juices with a wooden spoon. Gradually add the stock and scrape up all the caramelised juices in the tin. Simmer gently until it has reduced to a well-flavoured gravy. Season and strain into a gravy boat. Carve the lamb into slices and serve with the gravy and potatoes.