Preheat the oven to 350 degrees F.
With a small, sharp knife, make 12 (1 1/2-inch deep) slits about every 2 inches of the lamb. Wrap the anchovies around the garlic cloves and stuff into the slits.
In a large roasting pan set over 2 burners, heat the duck fat over medium-high heat. Add the lamb and sear, turning, on all sides, about 5 minutes. Remove from the heat and, in a small bowl, combine the rosemary, Essence, salt, and pepper, and rub evenly over the meat. Add the vegetables, and stir to coat with the fat. Roast until an instant-read meat thermometer inserted into the thickest part of the leg reads 130 degrees F for medium-rare, about 1 hour and 15 minutes, stirring the vegetables every 30 minutes.
Remove from the oven, transfer to a large platter, and cover loosely to rest for 15 minutes. Place the pan over 2 burners over medium-high heat. Add the wine and cook, stirring, to deglaze the pan and reduce by half. Add the stock and stirring, reduce again by half. Strain into a small saucepan through a fine mesh strainer, pushing down on the vegetables to extract as much liquid as possible. Return to medium heat, and stirring constantly, add the butter a piece at a time. Remove from the heat and cover to keep warm.
Carve the lamb, and if desired, serve with the roasted vegetables on the side. Serve with the gravy.
Yield: 8 servings