1. Marinate lamb for at least an hour or overnight.
2. BBQ over medium-high heat for 8 minutes, turning often so it does not char. 
3. Turn off one side of the BBQ and position the lamb on the side that is turned off. 
4. You will want to maintain a temperature of 300-350°F. Cover the grill and let cook for an additional 45-60 minutes (depending on how thick, and how many pounds the roast is), until a meat thermometer inserted into the thickest part of the roast registers 130°F (for medium rare). baste with the marinade a few times during cooking.
5. Transfer to a cutting board. Cover with aluminum foil and let rest for 10 minutes.