Preheat the oven to 350 degrees. Heat the olive oil in a large, deep Dutch oven with a tight-fitting lid over moderate heat until hot but not smoking. Season the lamb shanks with salt and pepper and cook, in batches if necessary, turning with tongs, until well browned, about 10 minutes each batch. Remove the lamb shanks with tongs to a plate and set aside.
Stir the onions, carrots, rosemary, bay leaves, and garlic into the dutch oven and cook, stirring, until the onions are very soft, about 5 minutes. Add 2 1/2 cups of the wine and deglaze the pan. Bring the mixture to a boil, reduce the heat, and simmer for 5 minutes. Taste for seasoning.
Return the lamb shanks to the pot and braise the mixture, tightly covered, in the oven for 1 hour. Uncover and braise for 2 more hours, turning the shanks every 30 minutes. Stir in the turnips and the remaining 1/2 cup of the wine, and continue to braise, in the oven for 30 minutes. Discard the bay leaves. Season to taste with salt and pepper.
While the lamb shanks are braising, make the gremolata: chop together the rosemary, orange zest, and garlic. Place each lamb shank on a serving plate, top with the vegetables, sprinkle with gremolata and serve immediately.