Excellent for Spring. Ask the butcher to butterfly the leg (removing the bone and opening the leg to make a flat piece of meat) and then back home spend five minutes with a sharp knife trimming up his or her handiwork. You will divide the leg into individual sections, following the muscles. There's a bit of waste, but you end up with pure meat and a judicious amount of fat, which helps baste the roasting meat, plus the end result is so much easier to carve than a whole leg with bones in it.
- Put the oil in a small pan over medium heat. When hot add the rosemary, ginger, garlic, 3 teaspoons of the salt, and pepper. Sauté for 1 minute, then remove from the heat. Allow the mixture to cool, then rub the paste into the lamb. Cover loosely and refrigerate for at least 4 and up to 24 hours.
- Take the lamb out of the fridge 2 hours before cooking. Position a rack in the upper third of the oven and preheat to 400°F.
- Set a large nonstick saucepan over high heat and heat the pan for 2 minutes. Keeping the heat high, sear the lamb on both sides in batches. Scrape any scraps of rosemary and ginger in the saucepan onto the seared meat after each batch.
- Arrange the lamb on a sheet pan and put in the oven to roast. Start checking the smaller pieces for doneness after 10 minutes. The largest piece will take about 20 minutes. If you have a meat thermometer, use it now. My thermometer broke about 6 years ago (memo from the school of antique cooking habits) so I use a metal trussing skewer. I stick the skewer into the center of the meat, wait 5 seconds, then touch the skewer to my cheek. If the metal is warm, the meat is done. This trick caused an uproar of consternation in the catering office, prompting a chorus of "How warm?" -- so maybe a meat thermometer is the way to go. Aim for 350°F.
- Allow the lamb to rest for 20 minutes. Thinly slice and serve warm or at room temperature.