Tuétano is bone marrow, obtained from the cow’s fibula and shinbone. This flavorful treat is prepared by grilling sliced bones until the marrow becomes soft. It is then removed with a spoon and commonly spread on a tortilla with a bit of salt, lime juice, chopped onion and cilantro. A caveat: bone marrow is not particularly healthful. But indulging every now and then, just as you occasionally reach for an extra slice of crispy bacon or two during Sunday brunch, isn’t going to hurt you.