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Heat oven to 180C/160C fan/gas 4. In a pan, cover the potatoes with cold salted water, bring to the boil and simmer for 5-10 mins until just cooked. Drain and allow to cool slightly, then cut into quarters.
Heat a frying pan over a medium-high heat until hot, then add 45ml of the oil. Add the potatoes, along with the carrots and garlic. Season, toss to coat with oil, then add half the butter and rosemary. Once there’s some colour on the potatoes, transfer everything to a roasting tin and cook in the oven for 25-30 mins.
Meanwhile, place an ovenproof frying pan over a medium heat and add the remaining oil. Season the lamb all over and, when the oil is quite hot, add it to the pan.
Fry the lamb for 1-2 mins each side, using tongs to press down gently to caramelise and render the fat. Season again and put in the oven for 5-7 mins until just cooked. When pressed with your fingers, the lamb should have a bounce to it. Remove from the frying pan and set aside in a warm place.
While the lamb rests, make the sauce. Add the red wine to the roasting juices in the frying pan over a medium-high heat. Reduce by two-thirds, then add the redcurrant jelly. Once the jelly has melted, add the lamb stock and reduce until it is the consistency of double cream.
Once the sauce is nearly reduced, put a separate pan on a low heat and melt the remaining butter. Add the spinach to wilt and season.
To serve, divide the potatoes between four warmed plates. Cut each piece of lamb into about five slices and place next to the potatoes. Arrange the carrots and spinach around the lamb. Spoon the sauce over everything and decorate with pieces of garlic and rosemary. Scatter a pinch of salt over each plate and serve.