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1
Preheat oven to 350u00b0F. Place lamb shanks on a large rimmed baking sheet; season generously with salt and pepper. Mix cinnamon, turmeric, cumin, and cardamom in a small bowl; massage spice mixture into lamb. Let rest 1 hour at room temperature or chill uncovered up to 12 hours (the longer, the better).
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2
If chilling, let shanks sit at room temperature 1 hour for even roasting.
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3
Heat 1/4 cup oil in a large pot over medium-high. Working in batches, cook lamb shanks until brown on all sides, 10-15 minutes. Transfer lamb to paper towels to drain.
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4
Wipe out pot and let cool slightly so that vegetables don't burn. Heat remaining 1/4 cup oil in pot over medium. Add onions, season with salt, and cook, stirring occasionally, until golden and softened, 8-10 minutes. Add thyme sprigs, garlic, and bay leaves and cook, stirring often, until fragrant, about 2 minutes. Sprinkle flour over and stir vigorously to evenly distribute. Cook, stirring often, until flour is absorbed, about 1 minute. Add wine; bring to a simmer and cook, stirring often, until thickened, about 2 minutes. Gradually stir in broth, pomegranate juice, and pomegranate molasses. Bring to a simmer and cook, stirring occasionally, until flavors meld, about 5 minutes.
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5
Arrange lamb shanks in a single layer in a large roasting pan. Pour in onion mixture and push lamb shanks down into liquid (about three-quarters of lamb shanks should be submerged; add more broth if needed). Cover pan with foil and cook lamb, turning occasionally, until meat is fork-tender and almost falling off the bone, 1 1/2-2 hours. Remove pan from oven and let lamb rest, covered, in liquid at least 30 minutes.
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6
Transfer lamb to a platter and tent with foil to keep warm. Strain braising liquid through a fine-mesh sieve into a medium saucepan; skim off fat from surface and discard. Add walnuts to braising liquid and bring to a simmer over medium-high. Cook until reduced by one-third, 15-20 minutes (sauce should be slightly thickened and velvety). Add butter a piece at a time, stirring after each addition until incorporated before adding more; sauce should look glossy. Return lamb to pot and cook, stirring to coat, until heated through. Taste and adjust seasoning with salt and pepper.
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7
Arrange lamb over polenta on a platter and spoon walnuts and sauce over; top with lemon zest.
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8
Lamb shanks can be braised 1 day ahead. Chill uncovered until cold, then cover and keep chilled.
