Heat the oil in a large heavy-based saucepan.
Add the garlic and onions and cook, covered, for 3-5 minutes, or until the onions are soft but not colored.
Add the mackerel, whiting and haddock or cod and cook, uncovered, over a moderate heat for 10 minutes, stirring from time to time.
Add the prawns and tomatoes.
Dissolve the saffron threads, if using, in the hot fish stock and add to the pan with the bay leaf parsley and salt and pepper to taste.
Stir and bring the mixture to the boil. Lower the heat and simmer, covered, for 15 minutes.
Then add the mussels and continue cooking for a further 10 minutes or until all the fish is cooked.
Discard the bay leaf, parsley sprigs and any mussels that have not opened.
Place the bread in a warmed soup tureen and ladle in the soup.
Sprinkle with the chopped basil before serving.