The secret to this recipe is the long, slow cooking on the bone together with the foil cover, and every time I make it I’m asked how it’s done. Cooking a shoulder of lamb for 3 hours makes the meat so tender that it almost pulls away from the bone in large pieces for serving, and the lamb juices give a delicious flavour to the potatoes baking underneath. The anchovies, garlic and vinegar work wonders, too. For a crowd, you can cook two shoulders in a bigger baking dish and increase the other ingredients. Cooking time will be the same.