Ceviche Verde

Cut fish into 1/2-inch dice, and place in a nonreactive container. Pour lime juice over fish, and sprinkle with 1 teaspoon salt and oregano; toss to combine. Refrigerate, covered, 6 to 8 hours.

Puree basil, parsley, mint, garlic, serrano, and the water in a blender until smooth but quite thick. With the motor running, add oil in a steady stream. Season with remaining 1 1/2 teaspoons salt.

Drain marinade from fish. Pour herb sauce over fish; toss to combine. Garnish with olives and onion, and serve.