A whole leg of lamb can weigh up to 9 lbs which tends to be more than a family can eat in several sittings so butchers have invented many new and exciting cuts but they can also be a bit confusing.
One way that we like to cut our whole legs is to take several steaks from the top end of the leg with gives what is called a "half leg sirloin end". Sirloin steaks (also called leg steaks) are perfect for grilling or pan frying when you want a nice tender piece of leg without having to cook up 5-6 lbs of meat. The half leg also takes off the shank which is best stewed since it is the most used portion of the leg. Osso Bucco is the recipe that made the shank famous.
The most important thing to remember about cooking a lamb roast is to not over-cook it. Lamb has such wonderful flavor on its own, and is so naturally tender, that it is bound to turn out well, as long as it is still a little pink inside. There is some debate over which method yields the best results - slow cooking at low heat the entire time, or searing first on high heat and then slow cooking. James Beard in his American Cookery prefers the slow-cook-low-heat method (he rubs the roast with salt and pepper and cooks it at 325°F the whole time.)Another point where there are wildly varying opinions is the internal temperature that constitutes "medium rare". I've seen references that range from 120° to 145°F. I pulled my lamb roast out at 130°F. As it rests the internal temperature continues to rise a few points as the meat continued to cook. We like lamb on the rare side of medium rare, and this roast was perfectly done to our taste. Clearly an accurate meat thermometer is essential and is one of the tools I can not be without in my kitchen.
Grass fed lamb cooks faster than grain fed lamb so keep checking the temperature to avoid over cooking.
Remember that any meat with a bone will cook more quickly than a boneless piece of meat so adjust the recipe accordingly.
Amount to Buy: For bone-in leg of lamb, allow about 3/4 pound per person; for boneless leg of lamb allow about 1/2 pound per person.