We’ve taken the classic approach of studding the lamb with garlic and coating in a generously seasoned rub. Giving it time to marinate is the key, then roast for a crispy exterior and tender, juicy inside.
Trim the excess fat from the lamb, leaving a 1/4- to 1/2-inch layer of fat. Using a sharp knife, make small slits, about 1/2 inch deep and 2 to 3 inches apart, all over the surface of the lamb. Fill each slit with a slice of garlic.
Grate the lemon zest into a small bowl (reserve the lemon). Add the rosemary, 1 tablespoon salt and 2 teaspoons pepper. Rub the mixture all over the lamb. Cover and refrigerate at least 4 hours or overnight.
Remove the lamb from the refrigerator and bring to room temperature about 1 hour before roasting. Preheat the oven to 425˚ F. Set a rack in a roasting pan and set the lamb fat-side up on the rack. Cut the reserved lemon in half and squeeze the juice over the lamb; drizzle with the olive oil and rub all over to coat. Add the lemon halves to the pan. Roast until the fat on the lamb starts to sizzle, about 20 minutes.
Reduce the oven temperature to 350˚ F and continue roasting the lamb until a thermometer inserted into the thickest part registers 130˚ F for medium rare to medium, 1 hour to 1 hour 15 minutes more. Remove to a cutting board and let rest 20 minutes before carving.
To carve leg of lamb, hold the lamb by the bone and slice the meat straight down along the bone.