Mix all the marinade ingredients into a large zip bag, then add the lamb and massage the marinade into the meat. Place in the fridge overnight (or min. 3 hrs)
Take the lamb out of the fridge an hour before cooking to bring to room temperature.
Once your BBQ is hot and ready to cook on, place the lamb on the grill fat side down and cook for approx 15 mins. Then turn over and cook for another 10 mins. I use a meat thermometer to help so you're aiming for 57°C / 135°F for medium-rare. Don't stress if thinner parts are cooked a little more.
Take off the grill, loosely cover with foil, and rest for at least 10 mins before serving.
Notes
Oven method:As above, then...
Preheat oven to 180°C/350°F.
Brown the lamb first in a frypan.
Roast for 22 – 25 minutes or until 57°C / 135°F for medium-rare.