Heat the oil in a large saucepan, add the onion and leek and fry gently for 5 minutes, stirring until softened. Meanwhile soak the saffron in 1 tablespoon boiling water.
Add the garlic and tomatoes to the pan and fry for 2–3 minutes then mix in the soaked saffron, wine, stock, thyme and some salt and pepper. Cover and simmer for 10 minutes.
Add the white fish, re-cover and simmer gently for 3 minutes. Add the mixed seafood, re-cover and simmer gently for 5 more minutes until all the fish is just cooked. Ladle into bowls and serve with toasted bread topped with spoonfuls of rouille (see below).