Lamb Shanks With Pomegranate and Walnuts

Name: Lamb Shanks With Pomegranate and Walnuts
Ingredients: 6 pounds lamb shanks (6–8 shanks, depending on size), trimmed; Kosher salt, freshly ground pepper; 1 1/2 teaspoons ground cinnamon; 1 1/2 teaspoons ground turmeric; 1 teaspoon ground cumin; 1/2 teaspoon ground cardamom; 1/2 cup olive oil, divided; 2 medium onions, thinly sliced; 6 sprigs thyme; 4 garlic cloves, crushed; 2 bay leaves; 2 tablespoons all-purpose flour; 1/2 cup dry red wine; 4 cups (or more) low-sodium chicken broth; 1 cup pomegranate juice; 1/2 cup pomegranate molasses; 2 cups walnuts, rinsed; 3 tablespoons unsalted butter, cut into pieces; Cooked polenta (for serving); 3 wide strips lemon zest, very thinly sliced lengthwise