This is from Bittman’s book, The Best Recipes in the World”, in which he says that in Rome, it would typically be made with bass or turbot. However, he also said it can be made with any fillet you like and chose to write up the recipe with cod (or you could, of course, use haddock). There are also variations in the notes for using some vegetables with it too, along with making it in a covered baking dish.
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