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

Heat 3 tablespoons of 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 10 minutes.

Place the fish in a steaming basket or colander and place the basket or colander inside the soup pot. There should be enough water in the pot to cover the fish. Cover the pot. Bring to a boil, reduce heat and simmer for 20 minutes, or until the fish are breaking apart. Remove the colander or steaming basket.

Place the zucchini and oregano sprigs in the pot. Place the fish basket/colander back in and cover the pot again. Bring back to a simmer and cook until the zucchini is tender, about 8 minutes. Remove the fish and oregano sprigs and discard. Season the soup with salt and pepper and stir in the remaining olive oil.

Serve with the rusks.