How to Make It
In a large enameled cast-iron casserole, heat 1/4 cup of the oil. Add the onion, leek, garlic, celery ribs and leaves, parsley and thyme and season with salt and pepper. Cook over moderate heat, stirring, until the vegetables are softened, 8 minutes. Add the lemon and wine and simmer over moderately high heat until the wine is reduced by half, 4 minutes. Add the stock and simmer over moderate heat until reduced by one-third, 5 minutes.
Season the halibut with salt and pepper and add it to the casserole. Cover and simmer over low heat for 5 minutes. Turn the halibut and add the mussels. Cover and cook over moderate heat until the mussels open, about 4 minutes. Season with salt and pepper and drizzle with the remaining 2 tablespoons of olive oil. Transfer the stew to a deep dish and serve right away with bread.