I like to cook the lamb on the medium-rare to rare side for tenderness. Total roasting time is approximately 10-15 minutes per pound, so plan on just over an hour to an hour and a half in the oven--longer if you're using a bone-in leg of lamb. Leg of lamb is best with a shorter marinade time, so marinade for just a couple of hours before roasting. Or, marinate the meat a day in advance for a couple of hours, remove the lamb and dry it off, then chill in the refrigerator for up to a day before roasting. You could also sear the meat in advance, then chill until ready to roast. The searing of the meat splatters a good bit, so you’ll want to avoid wearing your Sunday best while cooking this! Here we're using Michael Solomonov's perfect marinade that he uses for his famous lamb shoulder.
In a large bowl or saucepan (I use a saucepan because it’s narrow enough to immerse the entire leg of lamb in the marinade), combine the salt, sugar, fennel seed, peppercorns, allspice berries, and garlic with the water. Stir occasionally until the salt and sugar are dissolved, about five minutes.
Immerse the lamb in the marinade and marinate for 2- to 3 hours, refrigerated.
Adjust the oven rack to the lower third of the oven, and heat the oven to 375 degrees. Remove the lamb from the marinade and pat it dry with paper towel, removing any whole spices that may be lodged around the exterior of the lamb.
In an oven-safe low-sided roasting or sauté pan (such as Le Creuset), heat the butter and oil over medium high heat until very hot, about 3 minutes. Sear the lamb on all sides until the fat and exterior is caramelized and deep golden brown, about 4 minutes per side.
Transfer the pan with the lamb to the oven, and roast until a thermometer inserted into the center reads about 110 degrees. Take the pan out of the oven and brush the exterior of the lamb with pomegranate molasses.
Return the pan to the oven and continue roasting until a thermometer inserted into the center reads about 125-130 degrees, for medium rare. Remove the pan from the oven and brush the lamb again with pomegranate molasses.
Let the lamb rest until you are ready to slice and serve it, either warm (within 30 minutes) or at room temperature (up to an hour). Be sure to remove and discard the string before slicing.