Cut the fish fillets into 1½-inch pieces and season with salt. Heat 2 or 3 tablespoons of the olive oil in a frying pan and fry the pieces of fish. Lift out and place in a deep glass or porcelain bowl.
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 the olives and pour over the fish