AN IRISH STEW is perhaps the best-known lamb stew in the world, but a navarin d’agneau is the best-tasting one. Over the years I’ve taught this ragoût in many cities throughout the country, and I’ve often had students tell me that at least one member of their family wouldn’t eat lamb. They checked the recipe further and found the turnips. Now matters were much worse, for no one in their family ate turnips! Can the turnips be left out? Not if the stew is to be a navarin.