Prepare the barbecue for indirect cooking over medium heat (190°C to 230°C). If you are using a Weber Q barbecue, set up your barbecue with a convection tray and trivet.
Using a sharp knife, cut 10 to 12 small slits into the flesh of the lamb. Push a piece of garlic and a small sprig of rosemary into each slit.
Squeeze the lemon juice all over the lamb. Season with salt and pepper.
To estimate the cooking time, measure the meat through its thickest part and cook for 1 minute per millimetre for medium. For example if the roast is 90mm thick, cook for 90 minutes.
Roast the lamb over indirect medium heat for 1 ¼ to 1 ½ hours, with the lid closed, or until cooked to your liking.
Once the lamb has cooked, remove from the barbecue and leave to rest for 15 minutes before carving.
