Rub the oil all over the lamb and either brown it in a hot pan or throw it on the grill to get some colour. Meanwhile, toss all the potatoes and about half the garlic cloves into the bottom of your slow cooker.
Put the lamb on top of the potatoes, squish a few more cloves of garlic and rub it over the surface, then sprinkle with salt. Toss in a few sprigs of rosemary and pour some wine in around the potatoes, cover (if the bone sticks out, cover the lid with foil to seal in the heat) and cook on low for 6-8 hours. Carve the lamb and serve with the potatoes, finished with a squeeze of lemon, if you like.