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.
Wash and dry the fish, and cut into
For the cooking liquid, melt the butter in a large flameproof casserole or soup pot over medium heat. Add the onions, leeks, and garlic and cook, stirring often, until soft but not brown, 5 to 8 minutes. Add the stock, potatoes, bouquet garni, salt, and pepper and simmer until the potatoes are partially cooked, about 5 minutes.
Add the rich fish to the cooking liquid, pushing the pieces down into the liquid, and simmer for 2 minutes. Add the remaining fish and simmer until all the fish are nearly tender, 3 to 5 minutes longer. Add the sorrel or arugula and crème fraîche, shaking the pan so they mix into the liquid. Top with the mussels, cover, and continue simmering until the mussels open, 3 to 5 minutes.
Discard the bouquet garni, add the lemon juice, taste, and adjust the seasoning. Salt may not be needed, as the mussels are salty. Serve the cotriade directly from the pot, with the croûtes in a separate bowl. If you have guests, you may want to lift out the cooked mussels, shell them, and put the meats back in the stew before serving so they are easier to eat.
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