Trim the excess fat from the whole shoulder of lamb if necessary. Wipe the meat with damp paper towel and then cut small incisions into the meat on each side.
Combine cumin, salt, black pepper and saffron and rub mixture into the lamb, pushing it into the incisions. Cover and leave for 30 minutes to allow the flavours to penetrate. Place the lamb, fat side up, on a piece of cheesecloth, top with half the garlic cloves and tie the cheesecloth over the top.
Using a large saucepan onto which a steamer will fit, or the base of a couscoussier, fill it three-quarters full with water. If using a saucepan and steamer, check that the base of the steamer is at least 3 cm (1 1/4 inch) above the surface of the water. Cover and bring to a boil.
Line the base of the steamer with parsley stalks and the remaining garlic cloves. Place lamb on top, put folded strips of foil around the rim of the steamer and put the lid on firmly to contain the steam. Keeping the heat just high enough to maintain a boil, steam for 2-2 1/2 hours - do not lift the lid for the first 1 1/2 hours of cooking. The lamb should easily pull away from the bone when cooked. Lift it out of the steamer. Remove the cheesecloth.
Heat the oil in a large frying pan and quickly brown on each side for a more attractive presentation.