You can minimize this problem by trimming and tying the leg of lamb. You can do all this yourself or ask your butcher to do it. Here's what is 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 there) along with the end bit of shank meat, leaving a couple of inches of shank bone exposed.
- Make sure the butcher wraps the H-bone and the shank bone up for you so you can take them home to use for making a lovely lamb stock or sauce.
- 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.
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.
Seasoning the Leg of Lamb
Once the leg of lamb has been prepped in this way, the next step is to season it. The best and simplest way to season a leg of lamb is with garlic, rosemary, salt, pepper, and olive oil.
First, the garlic. The simplest method is to cut a garlic clove in half and rub the cut side of the garlic all over the roast. Another option is to slice about three cloves of garlic into slivers, then cut slits all over the outside of the roast and insert the garlic into the slits. Obviously, you'll get more garlic flavor this way!
For the rest of the seasonings, we make a paste of finely minced fresh rosemary leaves, olive oil, Kosher salt, and black pepper and then smear it around the roast. The proportions are about two tablespoons olive oil, 1 teaspoon of finely minced fresh rosemary, and 2 teaspoons each of Kosher salt and freshly ground black pepper; combine these ingredients in a small bowl to form a paste. If you're trimming and tying the roast yourself, rub some on the inner parts before you tie it. If the butcher tied yours up for you, you can untie it, rub the paste around, and then retie it—but in a pinch just smearing it 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 also take the chill out of the meat so that it cooks faster.
Roasting the Leg of Lamb
When it comes to the actual roasting, 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. In particular, it means you can't use a probe thermometer while roasting. Also, every time you open the oven door, you slow down the cooking.
Instead, try using a technique where you start the cooking at a high temperature, like 450 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 130 F. For a 6 to 8 pound leg of lamb, this whole process should take about an hour and a half.
Once the lamb reaches 130 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 let the meat hit its target temperature of 135 F, a perfect medium rare.
The meat closest to the bone may be slightly less done than the outer parts, but this is not necessarily a bad thing since there is usually at least one person around at a gathering who wants their meat cooked to a different variation of done-ness.