6 tablespoons extra-virgin olive oil
4 (5- to 6-ounce) tuna steaks
Salt and freshly ground black pepper
1 large onion, sliced (about 1 cup)
2 garlic cloves, minced
3 tablespoons salted capers, soaked in water for 10 minutes and drained
3 tablespoons minced pitted green olives
1/4 cup raisins
5 plum tomatoes, peeled, seeded, and chopped
1 cup fish stock or water, as needed
2 tablespoons fresh flat-leaf parsley, coarsely chopped
4 to 5 fresh basil leaves, torn
1/3 cup pine nuts, toasted
1. In a medium skillet over moderately high heat, warm the oil. Season the tuna with salt and pepper and sauté, turning once, until golden on both sides, 3 to 4 minutes total. Remove the tuna and set aside. Do not clean the pan.
2. In the same pan over low heat, sauté the onion, stirring frequently, until just tender, about 1 minute. Add the garlic, capers, olives, raisins, and tomatoes, and continue cooking, stirring, until heated through, about 2 minutes. If the tomatoes don't release much juice, add the fish stock or water to give the sauce some body.
3. Return the tuna to the pan and cover it with a tight-fitting lid. Cook over moderate heat until the tuna reaches the desired doneness, 2 to 3 minutes for medium.
4. Add the parsley and basil and stir to incorporate, then season to taste with salt and pepper. Serve the fish with the sauce spooned over the top; garnish with pine nuts.