Duck? Belgian endive? This sophisticated taco is an example of the kind of innovative cooking stirring things up in Mexico City. The upper tier of modern chefs in Mexico (as the city is called) generally have classical kitchen training, as well as experience working in Europe and, often, in Asia. They turn tradition on its head, mixing up preparations, adding Mexican flavors to other cuisines (and vice versa), incorporating new ingredients and rediscovering old ones, and generally having lots of fun. There are several steps to creating this nuevo taco, but everything up to the final steps may be done well ahead of time, and at the last minute you need only do the final duck cooking. Just before serving, warm the tortillas and assemble the tacos.