Meanwhile, discard any fat from surface of lamb shank mixture and bring to a simmer over medium heat. Cover, reduce heat to low, and simmer gently, occasionally turning shanks and stirring sauce, until heated through, about 20 minutes. If using large shanks, remove from pot; let cool slightly, then coarsely shred meat from bones. Return meat to sauce. (If you have 8 small shanks, you can serve them whole, setting them directly atop polenta.)