Sear seasoned lamb shanks in oil in a large casserole dish until golden-brown.
Sauté onions, celery, and carrots until soft, then add garlic and flour.
Deglaze with wine and stir in tomato purée, rosemary, bay leaf, and seasoning.
Return lamb to the pan, add stock, cover, and simmer gently for 3 hours.
Combine mint, lemon zest, parsley, and garlic for the gremolata and chill.
Boil potatoes until tender, steam cabbage, and warm milk with butter.
Mash drained potatoes with the milk mixture, stir in cabbage, and season.
Serve lamb shanks atop mash with sauce and a sprinkle of gremolata.