Leg of lamb is simple to cook yet makes an elegant, special-occasion presentation. While many recipes call for a fresh or thawed leg of lamb for roasting, you can cook it from a frozen state. The cooking time will be longer, of course.
Why Thaw Lamb
Recipes usually call for meat to be thawed before cooking for easier roasting. A large roast, for example, may overcook on the outside while the meat inside stays too underdone for safe consumption. Most lamb legs you get in the market weigh between 3 and 6 pounds, and are fine to roast from frozen. The largest legs that weigh up to 15 pounds should be thawed first to facilitate even cooking to a safe temperature.
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If you plan to sear the meat before roasting it, you do need to thaw the leg of lamb. If any breadcrumbs, vegetables or additions other than seasoning and spices are added, you may also want to thaw the meat first.
Cooking From Frozen
Heat the oven to 300 to 375 F. The lower the temperature, the longer the lamb will take to cook. Larger legs benefit from the lower temperature, though, because the outside will not cook too quickly.
Put in the oven and cook for about 50 percent longer than you would for lamb that was thawed. How long it cooks depends on the size of the leg and if it's boneless or bone-in. An instant read thermometer inserted into the thickest portion should reach 145 F according to the U.S. Food and Drug Administration. Temperatures of 160 F indicate the lamb is well done, and may be tough, chewy and unpalatable.
About halfway through the cooking process, toss vegetables to roast at the bottom of the pan; red onion, carrots and potatoes are options. Lay fresh rosemary and lemon slices on top of the lamb at the halfway point too, if desired.
Never cook leg of lamb from frozen in a slow cooker. It will spend too much time at low temperatures, which encourages the growth of harmful bacteria.