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The night before, trim the fat from the lamb then mix with the yogurt and rosemary leaves. Cover and put in the fridge to marinate overnight.
The next day, trim the stalk off a romanesco so that it sits upright on a board. Slice 5cm off each side with a bread knife (keep these off-cuts to make the couscous). Slice the rest of the romanesco into three thick slices. Repeat with the other romanesco. Oil a large, non-stick baking tray with 1 tbsp vegetable oil. Lay the romanesco slices in a single layer and drizzle with a little more vegetable oil. Season with salt and scatter over the rosemary stalks, ready to roast.
Finely chop or pulse the romanesco off-cuts in a food processor until they are the texture of couscous. Mix the finely chopped walnuts into the couscous with the olive oil, and season with salt.
Heat oven to 200C/180C fan/gas 6. Wipe the marinade off the lamb and season with sea salt. Heat 2 tbsp vegetable oil a large ovenproof frying pan. Fry the lamb until golden all over, then roast in the oven for 8-12 mins, depending on how rare you like it. Put the romanesco in the oven at the same time. Once the lamb is ready, remove from the oven, flip the romanesco slices over so they char on both sides, and cook for a further 10 mins. Cover and rest the lamb in the pan for 10 mins.
Meanwhile, simmer the stock in a wide saucepan with 1 tbsp of the walnut pickling vinegar from the jar for 10 mins until it forms a jus.
Slice the lamb and serve with the romanesco slices, some couscous, half a walnut and the jus.
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