Trim roast (we used a leg of lamb). Weigh and calculate cooking time-for lamb allow 25 to 30 minutes per 500g.Place on a trivet (roasting rack) in a baking dish. This allows the meat juices to drain form the roast.rim roast (we used a leg of lamb). Weigh and calculate cooking time-for lamb allow 25 to 30 minutes per 500g.