Preheat the oven to 325 degrees . In a large Dutch oven, heat the olive oil over medium-high heat. Season the lamb shanks with 2 teaspoons salt and 1 teaspoon pepper, add to the pot and cook, turning, until browned, about 10 minutes. Add the onions and cook until softened, 4 to 5 minutes. Add the beef broth, grapefruit juice, rosemary sprigs and 2 tablespoons garlic and bring to a boil; remove from the heat.
Cover the pot, transfer to the oven and cook for 45 minutes. Turn the lamb shanks and cook for 30 minutes more. Add the carrots and cook until the carrots are tender and the lamb is almost falling off the bone, about 45 minutes.
Meanwhile, in a small bowl, stir together the grapefruit peel, chopped rosemary and the remaining 1 tablespoon garlic. In a medium saucepan, bring 4 cups salted water to a boil. Gradually whisk in the polenta and cook over medium-low heat, stirring constantly, until it begins to pull away from the sides, about 15 minutes. Remove from the heat and cover to keep warm.
Remove the lamb shanks from the oven and stir in the grapefruit peel mixture; let stand for 10 minutes. Divide the polenta among 4 bowls and top with the lamb shanks, carrots and braising liquid.