A leg of lamb is made up of three different muscles; the knuckle, the topside and the silverside. To bone the lamb leg you will need to use a method called tunnel boning. The first thing you will need to do is remove the aitch bone at the wide end of the leg. This is done by following the contour of the bone with the point of your knife and cutting through the cup joint.
Next you need to remove the meat from around the end of the leg bone that was connected to the cup joint. This should be done with the top 1cm of your knife in a scaping action, pushing the meat back down to expose the bone. You then need to move to the other end of the leg and cut either side of the leg bone (following the edge of the shank or shin) down to the knee joint, but be careful not to cut through it. Then scrape the meat off the bone as before.
Now you need to lay the leg flat on the bench and twist the bone until the meat releases and then pull it clear of the leg. Once this is done all you need to do is remove the fat kernel which is located between the topside and the silverside muscle and you are finished.