1. Cut the fish fillets into 1 1/2 inches pieces and season with salt. Heat 2-3 tablespoons oil in a frying pan and frythe fish. Drain and place in a deep bowl.
2. Combine the onions, garlic, bay leaves, peppercorns and vinegar in a saucepan and simmer until the onion is tender. Cool. Add the remaining olive oil and olives and pour over the fish.
3. Cover the bowl and refrigerate for at least 24 hours before serving. Serve cold, garnished with lettuce leaves.