Place the leg, fat side down, on a cutting board. Using your fingers, locate the bone. It will be about an inch under the skin and runs from one end of the leg piece to the other; you should see the joints at either end.
With a sharp boning knife, cut straight down the length of the leg from bone joint to bone joint. Carefully cut the meat away from the bone, working as closely as possible to the bone.
Run the tip of your knife under the bone and cut back toward one of the joints. When you get to the joint, cut around it and loosen it from the meat. Now take your knife and repeat in the other direction until you reach the other joint. Cut around it and remove the bone.
At either end where you removed the joints, there will be firm connective tissue. Using your boning knife, carefully remove that tissue.
If you are planning to butterfly the leg and grill it, lay the deboned leg flat on the board, meat side up. There will be a larger chunk of meat on one side and a smaller chunk on the other. Slice into the thicker piece of the leg to divide it in two horizontally, without cutting through the skin completely. Open out that piece of meat so that you have one somewhat evenly flat leg piece.
Now roll the leg back together, flip it over and trim away the excess fat and silver skin on the top side.
If you are putting the roast on a spit or roasting in the oven, tie the roast in two or three places along its length; tie once at 90 degrees to the previous ties along the width of the roast. If you are grilling the butterflied piece, flatten it out, season both sides and it is ready for the barbecue.
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