Here, lamb is accentuated by briny olives and aromatic herbs.
To prepare the lamb: Heat oven to 450 degrees. Rub the lamb with 1 tablespoon of oil, and season it with 1 tablespoon salt and 1 1/2 teaspoons pepper.
In a medium saucepan over medium-high heat, bring the stock and wine to a boil; allow to reduce for 10 minutes. Meanwhile, warm the remaining 2 tablespoons of oil in a large Dutch oven over medium heat. Add the onions and cook, stirring occasionally, until soft, 7 to 10 minutes. Stir in the carrots and parsnips, 1/4 teaspoon salt, the remaining 1/4 teaspoon pepper, rosemary, sage and bay leaf. Turn off the heat and add just enough stock to cover the vegetables.
Place the lamb, fatty side up, on top of the vegetables. Transfer the pot to the oven, uncovered, and cook for 25 minutes. Then add the remaining stock, cover the pot and reduce heat to 325 degrees. Cook for 1 1/2 hours, at a bare simmer, reducing heat if necessary, then turn the lamb over. Cook 1 1/2 hours longer and turn the lamb over again. Uncover the pot and stir in the olives. Cook another hour, turning the lamb after 30 minutes. At this point the lamb should be soft enough to cut with a serving spoon. If not, cover the pot and continue to cook until it is.
After the lamb has cooked for about 3 hours, prepare the celery root purée: In a large saucepan, combine the celery root, peeled garlic cloves and bay leaves. Pour in 12 cups water and 2 tablespoons of kosher salt. Over medium-high heat, bring to a boil; reduce heat and simmer until tender, 20 to 25 minutes. Drain, discard the bay leaves and transfer the celeriac and garlic to a food processor. Add the butter and nutmeg; process until very smooth. Taste and add more salt if necessary. Keep warm.
Just before serving, mash the finely chopped garlic and the remaining 1/4 teaspoon salt to form a paste. Stir it into the lamb’s pan juices. To serve, make a bed of celery root purée on each plate. Cut the lamb with a serving spoon, and lay some of it over the celery root, along with some vegetables and pan juices.