Each year Bonito del Norte tuna swim north to Spain's Bay of Biscay where they spend the summer fattening up in preparation for their return to warm southern waters for the winter months. The ventresca, or belly, is the silkiest, most coveted part of the tuna. This delicate ventresca, cooked within a day of peak season harvest and hand-packed in olive oil has a mellow flavor and unctuous, almost buttery texture. You'll find Conservas Ortiz's fish on the menus at the best restaurants in the US and Spain. Barcelona-born Marc Vidal of Boqueria in New York City recommends it lightly dressed with potatoes and seasonal vegetables.