Season the meaty sides of each rack with 1 tablespoon of the Essence. Heat the butter in a medium skillet over medium-high heat. Add the onions, celery, shallots, 1/4 teaspoon of the salt, and pepper, and cook, stirring, for 1 minutes. Add the garlic and cook for 30 seconds. Add the apricots, raisins, and prunes, and cook for 2 minutes. Add the Port and cook until almost all the liquid has evaporated, about 5 minutes. Add the mint and walnuts, and stir well to combine. Remove from the heat and let cool for 10 minutes. (If desired, the stuffing may be made 1 day ahead and refrigerated, tightly wrapped.)
Preheat the oven to 400°F.
With a thin, sharp knife, cut into the middle of the lamb loin going as deep as you can. Turn the lamb and cut into the center from the other end, to make the 2 ends meet as one long slit. Insert a long handled wooden spoon into the slit and push it through the hole on the other side. Gently work the spoon handle to enlarge the hole to 1/2- to 3/4-inch in diameter. With your fingers, work the fruit stuffing into the center of the meat so that the hole is filled with about 1/4 cup of the stuffing. Repeat with the remaining racks and stuffing.
Heat a heavy skillet over medium high heat. Add one rack to the pan and sear until brown on all sides, about 7 minutes.
Combine the breadcrumbs, olive oil, remaining tablespoon of Essence, and remaining 1/2 teaspoon of salt, in a shallow bowl.
Rub each rack with 2 tablespoons of the mustard, then dredge in the seasoned breadcrumbs, coating both sides evenly. Place the lamb on a roasting pan meaty-side down, and cover the bones with aluminum foil to prevent them from burning as they cook. Roast the racks of lamb 20 – 25 minutes for medium-rare. While the racks are roasting, make the sauce.
SAUCE FOR STUFFED RACK OF LAMB:
Reserve 2 tablespoons of the dry fruit stuffing. In a 1-quart saucepan set over a medium-high heat add the olive oil. Add the shallots and garlic to the pan and sweat for 1 minute. Add the onions to the pan and sweat for 2-3 minutes. Add the dry fruit stuffing to the pan and sweat for 1 minute and deglaze with the red wine. Reduce the wine till nearly evaporated, and add the stock and the walnuts to the pan and reduce by 30%. Add the chopped sage to the sauce and season with the salt and black pepper. Swirl the cold butter into the sauce off the heat, and serve immediately over the lamb.
Yield: approximately 1 1/4 cups of sauce