Crumble the saffron into a small bowl and add 2tbsp boiling water. Leave to stand. Meanwhile, heat the oil in a large saucepan. Add the onions and garlic and cook gently for 15mins until soft.
Add the tomatoes and fish stock. Bring to a gentle simmer. Add the saffron and liquid, season and simmer uncovered for 15mins.
Remove skin and any bones from the fish and cut into large pieces. Pat dry on kitchen paper and season. Wash the mussels under cold running water. Discard any broken shells; tap any open shells and discard any that don't close.
Stir the prawns and mussels into the stew, then layer the white fish pieces on top. Gently push the pieces into the juices, then cover and cook over the lowest heat for 10mins until the fish is just cooked through. Check the seasoning and sprinkle with the basil.
Ladle into dishes and serve immediately with crusty bread.