Despite its 3,000-year history of catching wild bluefin tuna, Spain’s tuna culture has largely been limited to canned and salted tuna products. This luscious dish, a staple of southern Spain, is one of the exceptions. Slideshow: More Tuna Recipes
In a large skillet, heat the olive oil over moderately low heat. Add the garlic, onion, bay leaves, dried oregano and paprika and cook, stirring occasionally, until the onion is very soft but not browned, about 25 minutes. Season with salt.
Stir in the vinegar and 3 tablespoons of water and bring to a simmer. Add the tuna in a single layer over the onions, cover and simmer until the fish is just cooked through, about 5 minutes. Spoon the sauce over the tuna, garnish with fresh oregano, if desired, and serve.