Preheat oven to 190°C / gas mark 5.
1) Place the lamb in a resealable plastic bag. Combine the marinade ingredients in a glass bowl or measuring cup. Pour the marinade over the lamb, seal the bag, and refrigerate for at least one hour or overnight.
2) Once marinated, place the lamb in a cast iron pan with olive oil and sear on all sides. Cover pan with aluminium foil and place in oven.
3) Roast the lamb until done, for about 2 to 2 1/2 hours. The internal temperature should reach 80°C on an instant-read thermometer.