Heat the oil in a large, heavy-based pot. Add the carrots, celery and onions, cover the casserole and cook over a low heat for 5 minutes.
Add the garlic and potatoes and continue to cook, uncovered, for a further 5 minutes, stirring occasionally.
Pour in the stock and stir in the saffron. Cover and simmer for 10 minutes, or until the vegetables are just beginning to soften.
Meanwhile, skin the fish, remove any bones and cut it into 4 cm (1½ in) chunks. Add the fish, beans and wine to the pot, cover and simmer for a further 7 minutes.
Dissolve the cornflour in the milk, then gently stir it into the casserole, along with the prawns. Allow the sauce to bubble gently and thicken slightly, taking care the fish does not break up and the prawns are warmed right through. Season to taste, stir in the herbs and serve.