The trick to roasting a leg of lamb is ensuring that it cooks evenly. This can be tricky since a bone-in leg of lamb has a wide end and a skinny end, and the skinny end naturally cooks faster. You can minimize this problem by trimming and tying the leg of lamb or having your butcher do it.
Trim and Tie the Leg of Lamb
Whether you have your butcher trim and tie your leg of lamb or do it yourself, there are a few steps involved:
- Remove the hip bone, sometimes called the H-bone because it looks like the letter H, along with the attached tailbone.
- Remove the hinged end of the shank bone (if it's still attached) along with the end bit of shank meat, leaving a couple of inches of shank bone exposed.
- Trim away the excess fat and pull the skin off the outer part of the leg.
- Remove the lymph node and connective tissue from the little seam between the two main lobes of meat on the leg.
- Tie the leg up with butchers twine. This makes the leg into a tight and compact roast, enabling it to cook as evenly as possible.
If you're having a butcher trim your leg of lamb (highly recommended), make they wrap the H-bone and shank bone up for you. They can be used later to make a lovely lamb stock or sauce.
Depending on where you shop, you might see something called a semi-boneless leg of lamb, which is basically a leg of lamb prepared as described above. The only difference is that the lymph node isn't always removed. This isn't a huge deal, but it does have a strong flavor and gristly texture, so if you're not sure just ask the butcher.
Season the Leg of Lamb
The traditional seasoning for a leg of lamb is garlic, rosemary, salt, pepper, and olive oil.
To add a touch of garlic flavor, cut a garlic clove in half and rub the cut side of the garlic all over the roast. Alternatively, slice a few cloves of garlic into slivers. Cut small slits all over the outside of the roast and insert the garlic into the slits. This will result in more garlic infusion.
Make a paste of finely minced fresh rosemary leaves, olive oil, Kosher salt, and black pepper and smear it on the roast. If you're trimming and tying the roast yourself, rub some on the inner parts before you tie it. If the butcher tied it up for you, you can untie it, rub the paste around, and then retie it—but just smearing the paste on the outside is fine, too.
Once you've tied and seasoned the roast, let it sit at room temperature for about 30 minutes. This will let the flavors absorb into the meat and take the chill out of the meat so that it cooks faster.
Roast the Leg of Lamb
Some recipes will have you turn the roast every 20 minutes so that it cooks evenly. This isn't a bad technique, but it is kind of a hassle and means you can't use a probe thermometer while roasting. Plus, every time you open the oven door you slow down the cooking.
Instead, try using a technique where you start at a high temperature like 425 F for about 20 minutes, then lower the heat to 325 F and cook it the rest of the way. For a leg of lamb, that means until the probe thermometer reads 140 F for medium-rare (150 F for medium). For a 6 to 9-pound leg of lamb, this whole process should take about two to three hours.
Once the lamb reaches 140 F, take it out of the oven, tent it with foil, and let it rest for about 20 minutes. This will let the juices redistribute throughout the meat and also allow the meat to hit its target temperature of 145 F.