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Heat the oven to 210C/190C fan/gas 7. Generously season the lamb all over and heat the butter in a large ovenproof frying pan over a medium heat. Brown the lamb all over for 10-15 mins, especially the fat side. Remove and set aside on a plate.
Put the pan back on the heat and fry the onion for 5-7 mins until softened, then add the garlic, anchovy (if using) and rosemary, and fry for 2 mins more. Carefully arrange the potatoes in the pan with the olives, season, then turn them over so they’re fully coated in the mixture, then spread out in an even layer. Pour over the stock, then put in the oven for 30 mins until the potatoes start to crisp at the edges.
Sit the lamb on top of the potatoes, fat-side up, and drizzle over any of the juices from the plate. Put the pan back in the oven for 20 mins for lamb that’s rare, 25 mins for lamb that’s pink but cooked through, and 30 mins for well done. When the lamb is done to your liking, lift onto a carving board and leave to rest for 10 mins. During this time, put the potatoes back in the oven to make them extra crisp. Cut the lamb into chops and serve with the potatoes, the cavolo nero and parsley & caper dressing (see 'goes well with', below).