As happens so often with things like this, it all started with just one word, and the word was buey. That's what the Spanish call an ox.

Three years ago, my wife and I were driving around Basque country in northern Spain, and we stopped at an asador, a restaurant specializing in grilling and roasting meat over aromatic wood and charcoal fires. When we had polished off a beautiful chuleta (a chop, which is what the Spanish call a bone-in rib steak), the owner-chef boasted that his meat had come from an eight-year-old buey from the region of Galicia in northwest Spain; the animal had worked all its life and had been fed both grass and grain throughout. As the chef was speaking in Basque and his sous chef was translating into French, I was sure I had misunderstood. And so I spent the next 10 minutes cross-examining him until I became convinced at last that, yes, I had stumbled upon a seriously major and novel gastronomic phenomenon, both revolutionary and earthshaking, and probably outmoded at the same time.

I had reasons for deep suspicion. Everybody knows that steers and cows become incredibly tough as they grow older. Their flesh darkens and acquires a strong, strange flavor. Their well-exercised muscles are shot through with connective tissue, which itself is intolerably tough. That's why, in this country, steers are slaughtered before they reach the age of 30 months, and usually much earlier. (Our cows are slaughtered when they can no longer produce calves or milk in an economical fashion.) Besides which, Spain is not the first country that comes to mind when you think of eating your way through a succulent steak. The city of Segovia is famous for its roast suckling pigs, and Valladolid is the place to go for spit-roasted baby lamb. But nobody has ever heard of eating a grilled ox.

I read everything I could find, which was nothing, and questioned everyone I could think of. My curiosity mounted into a preoccupation, and then into an obsession. Relief finally arrived a year later in the form of Lydia Itoi, a Japanese-American friend who writes about food and travel for the European edition of Time. Lydia introduced me to two of her Spanish friends who had been to a remarkable rural restaurant named Bodega El Capricho ("The Whim"), which specializes in serving the meat of aged bueyes and even cows. One of them, Pedro Espinosa, is an IT executive who also writes a weekly restaurant review for El Mundo, Spain's second-most-popular newspaper; his friend, Rogelio Enríquez, had blogged about the same restaurant on his own gastronomy site, pistoynopisto.com. Over the following six months we formulated a plan. I would fly from New York to Bilbao, where I would meet up with Pedro. After three days of dining at several of Spain's leading asadores, we would drive most of the way across northern Spain and eat a buey at El Capricho. (For your information, an ox is a castrated male bovine more than four years old; a steer is also male and castrated, but younger than four; a bull has not been castrated. A cow is a female bovine that has calved at least once; a heifer is a female who hasn't.)