Searing the Lamb Shanks: Heat the olive oil in a large heavy-bottomed Dutch oven over medium-high heat. Generously season the lamb shanks with salt and pepper. Add the lamb pieces in batches to avoid overcrowding and sear until they develop a rich, golden-brown crust on all sides. Once browned, transfer them to a plate and set aside.
Building the Flavor Base: Lower the heat to medium and add the unpeeled garlic cloves, chopped celery, half of the chopped carrots, and onion to the pot. Stir occasionally and cook for about 10 minutes, allowing the vegetables to soften and caramelize to a golden color, which will deepen the stew’s flavor.
Tomato Paste and Wine Reduction: Stir in the tomato paste and cook for 2 minutes, letting it deepen in color and flavor. Pour in the red wine, raising the heat to high, and boil until the liquid thickens into a syrupy consistency—this should take roughly 5 minutes.
Braising the Lamb: Return the browned lamb shanks to the pot. Pour in the chicken stock and add the strips of orange zest. Bring the mixture to a gentle simmer, then cover the pot, reduce the heat to low, and let the lamb braise for approximately 3 hours. Occasionally turn the shanks to ensure even cooking until the meat is tender and pulls away easily from the bone.
Roasting the Root Vegetables: While the lamb braises, preheat your oven to 350°F (175°C). Toss the rutabaga, turnip, parsnips, and the remaining carrot pieces with olive oil, salt, and pepper in a large baking pan. Roast in the oven for about 60 minutes, stirring every 20 minutes, until the vegetables are tender and caramelized.
Preparing the Gremolata: In a small bowl, combine minced garlic, chopped parsley, and lemon zest. Set this aromatic garnish aside to add a fresh, zesty note to the stew at the end.
Finishing the Sauce: Remove the lamb shanks from the braising liquid and set them aside. Pass the cooking liquid through a coarse strainer into a separate bowl, pressing the vegetables to extract all the flavorful juices. Discard the solids and skim off any fat that rises to the surface. Return the strained sauce to the pot and bring to a boil over medium-high heat. Season with salt and pepper to taste.
Combining and Serving: Remove the lamb meat from the bones and add it back to the pot with the roasted root vegetables. Simmer briefly until everything is warmed through and well combined. Serve the stew hot, garnished with the fresh gremolata for a vibrant finish.