Take the roast out of the fridge 3 hours or so before you put it on the bbq that helps a lot with having rare meat in the middle. Season it generously with your favorite seasonings. Sea salt, pepper, Montreal steak spice and prime rib seasoning is what I use. I put it in a tin foil roasting pan and cover with tin foil. Turn it about every 20 minutes. Takes about 2 hours for an 8 pound roast. Use a meat thermometer. I take it off to rest when center of roast is 140°. Trial and error = perfection.
