Score the lamb across the top of the joint with a sharp knife then rub with the olive oil. Rub all of the spice in and then refrigerate for 2 to 24 hours, preferably the latter.
Pre-heat the oven to 220ºC. Place the onions, red wine vinegar and stock in the bottom of baking tray and then lay the lamb on top. Place in to the oven for 30 minutes then reduce the temperature to 120º for the remaining 5 ½ hours.
During this time mix all of the slaw ingredients together, place in a bowl and cover until required. After a couple of hours this cabbage and onions will be nice and soft, perfect for this dish and for any BBQ.
If the lamb browns too much during cooking then you can cover in tin foil however this rarely happens.
Once cooked, enjoy the satisfaction of how soft and juicy the lamb is. Using two forks or knife pull apart and then enjoy with soft tortillas, homemade mayonnaise and a little slaw.