Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the lamb into the slow cooker pot, add the rosemary on top and tuck the onion wedges around the sides of the joint.
Spoon the redcurrant jelly into a small saucepan and add the wine, stock and a little salt and pepper. Bring to the boil, stirring so that the jelly melts, then pour over the lamb. Cover with the lid and cook on high for 7–8 hours or until a knife goes into the centre of the lamb easily and the meat is almost falling off the bone.
Lift the joint out of the slow cooker pot and put it on to a serving plate with the onions. Discard the rosemary sprigs and pour the wine and stock mixture into a jug to serve as gravy. Carve the lamb on to plates and serve with steamed green vegetables and baby potatoes or crushed new potatoes with rosemary cream (see below).