How to Make It
In a large enameled cast-iron casserole, heat the olive oil until shimmering. Add the onion, fennel and garlic, season with salt and cook over moderately high heat, stirring occasionally, until lightly browned, about 7 minutes. Add the wine, bring to a boil and cook, scraping up any browned bits from the bottom, until slightly reduced, about 1 minute.
Add the potatoes, tomatoes and their juices, clam juice, orange juice and orange zest and bring to a boil over high heat, then simmer over moderate heat for 15 minutes.
Add the shrimp and cod and cook until almost white throughout, about 3 minutes. Add the mussels, cover and cook until they open, about 3 minutes. Discard any unopened mussels and the orange zest. Season with salt. Ladle the stew into bowls, top with the parsley and crème fraîche and serve immediately.