Leek and sorrel add an agreeably acid bite to this Breton stew of white fish, dramatically topped with salty black mussels. Cotriade is fortified with potatoes and made with white fish, like cod, haddock, hake, or whiting, plus a bit of rich fish, such as eel or mackerel. American alternatives are bluefish, red snapper, and flounder. Arugula can take the place of sorrel.