A cooked, well-seasoned leg of lamb can impress even the most finicky of guests with its rich flavour and tenderness. You can roast a leg of lamb many ways, all of which may alter your cooking time slightly. While using a thermometer helps you achieve the desired completeness according to the USDA guidelines, each cook and chef vary in opinions about how to get the perfect result. Many people rely strictly on cooking time and don't use a thermometer, but both methods are valid.
Cooking your leg of lamb to a medium-rare finish, with a pronounced pink hue to the meat, means the internal temperature should reach 63 degrees C/145 degrees F. With your oven preheated at 163 degrees C/325 degrees F, you will cook the lower leg half (called the shank) for 25 to 30 minutes per pound. The upper half (called sirloin) should cook for 25 minutes per pound. This is about three to four pounds of meat. The whole leg with the bone weighing between five to seven pounds should be cooked for 15 to 20 minutes per pound. Larger legs need to be cooked longer: 20 to 25 minutes per pound. A boneless leg roast should be cooked for 20 minutes per pound.
Medium for leg of lamb means the inside meat is barely pink, with the internal temperature reaching 70 degrees C/160 degrees F. So for the shank half, you will cook the same three- to four-pound specimen for 30 to 40 minutes per pound at 163 degrees C/325 degrees F. Cook the sirloin half for 35 minutes per pound. A four- to seven-pound boneless roast should be cooked at the same temperature for 25 minutes per pound. A whole leg (bone-in) up to seven pounds should also be cooked for 20 to 25 minutes per pound.
A well-done leg of lamb has no pink on the inside. When well-done, the centre portion reaches 76.7 degrees C. To achieve a well-done finish, add between five and 10 minutes cooking time per pound in a 325-degree preheated oven to the cooking time for medium. Thus, a whole leg up to seven pounds will cook for 25 to 30 minutes per pound. Larger legs up to nine pounds will cook longer, for up to 35 minutes per pound. Shanks up to four pounds will need 35 to 45 minutes per pound to be well done. Sirloin halves should be cooked for 45 minutes per pound. A boneless roast up to seven pounds will need 30 minutes per pound.