PreparationMake salad: Mince and mash garlic to a paste with 1/2 teaspoon salt. Transfer to a bowl and whisk in anchovy paste, vinegar, and 1/4 teaspoon pepper. Whisk in oil. Toss with remaining salad ingredients. Let stand, stirring occasionally, while fish roasts. Roast fish: Preheat oven to 425°F with rack in middle. Oil a 1 1/2-to 2-quart gratin or other shallow baking dish. Rub flesh sides of fish with 2 teaspoons oil (total) and season with 3/4 teaspoon salt and 1/4 teaspoon pepper (total). Divide onion slices and oregano sprigs into 2 portions and sandwich each portion between 2 fillets, skin sides out. Tie with kitchen string crosswise at 2-inch intervals and transfer to baking dish. Score skin on top in several places with a sharp knife and drizzle with remaining 4 teaspoons oil. Roast fish until just cooked through, about 15 minutes. Cut off string and cut sandwiched fillets in half crosswise. Serve topped with salad. Cooks' note:Salad can be made 6 hours ahead and kept at room temperature.
PreparationMake salad: Mince and mash garlic to a paste with 1/2 teaspoon salt. Transfer to a bowl and whisk in anchovy paste, vinegar, and 1/4 teaspoon pepper. Whisk in oil. Toss with remaining salad ingredients. Let stand, stirring occasionally, while fish roasts. Roast fish: Preheat oven to 425°F with rack in middle. Oil a 1 1/2-to 2-quart gratin or other shallow baking dish. Rub flesh sides of fish with 2 teaspoons oil (total) and season with 3/4 teaspoon salt and 1/4 teaspoon pepper (total). Divide onion slices and oregano sprigs into 2 portions and sandwich each portion between 2 fillets, skin sides out. Tie with kitchen string crosswise at 2-inch intervals and transfer to baking dish. Score skin on top in several places with a sharp knife and drizzle with remaining 4 teaspoons oil. Roast fish until just cooked through, about 15 minutes. Cut off string and cut sandwiched fillets in half crosswise. Serve topped with salad. Cooks' note:Salad can be made 6 hours ahead and kept at room temperature.