Season the fish with a little salt and pepper and set aside until ready to use.

Heat 3 tablespoons of the olive oil in a large, wide pot and cook the onions, carrots, and celery over medium heat, covered, until wilted. Season lightly with salt and pepper. Stir in the tomato paste. Add enough water to come about half way up the pot. Add the potatoes, cover the pot and cook for 15 minutes.

Stir in the zucchini and cook for a few minutes, until tender. Add the fish pieces and/or whole fish to the soup, wrapped in cheesecloth, to prevent the bones from falling into the broth as the fish cooks.

Add water if necessary. There should be enough water in the pot to cover the fish. Cover the pot. Bring to a boil, reduce heat to medium-low, and simmer for 15 - 20 minutes, or until the fish are tender and falling off the bone. Remove the fish and set aside, covered, to keep warm. Using an immersion blender, puree about a third of the soup in the pot to create a thick broth. Season with oregano and gently stir in the olives. Heat all together for 2 - 3 minutes.

Adjust the flavor of the soup with additional salt and pepper and stir in the remaining olive oil.

Serve the soup with fish pieces in individual bowls or one large soup tureen. You can add rusks or grilled bread to the soup or to individual bowls if desired. Drizzle with additional olive oil if desired, too!