Preheat the oven to 350. Season the lamb shanks generously all over with salt and pepper. In a dutch oven over medium-high heat add the olive oil and heat until almost smoking. Add the lamb shanks and sear until browned on all sides. Remove the shanks from the pan and lower the heat to medium. Add the carrots, onion, and celery and sweat (try not to brown) for about 10 minutes or until tender. Add the garlic and rosemary and cook for another 1-2 minutes. Add the wine and reduce by half, scraping the bottom of the pot with a wooden spoon to dislodge any delicious browned bits. Stir in the tomatoes, add the chicken stock, bring everything to a simmer, cover the pot and place it in the oven for about 3 hours or until the lamb shanks are fork tender and easily pull off the bone. When ready, remove the lamb shanks from the liquid and pull the meat of the bone and roughly shred with a couple of forks. Meanwhile, simmer the braising liquid and skim the fat with a ladle. Add the shredded meat back to the braising liquid and simmer until the liquid has thickened (reduce by about half). Serve atop the polenta and garnish with the mint gremolata.