• 1 pound/450 g rich fish fillets, without skin
• 1 pound/450 g white fish fillets, without skin
• 1½ pounds /675 g mussels
• 1 pound/450 g sorrel or arugula, stems removed
• 2 tablespoons/30 g butter
• fried croûtes made with 1 baguette, sliced, fried in 4 tablespoons/60 g butter, and rubbed with a cut garlic clove

Cooking Liquid

• 2 tablespoons/30 g butter
• 2 onions, chopped
• 2 leeks, white and green parts, chopped
• 2 garlic cloves, finely chopped
• 1 quart/1 liter fish stock
• 1 pound/450 g potatoes, peeled, quartered, and thinly sliced
• 1 bouquet garni
• salt and pepper
• 1 cup/250 ml. crème fraîche or heavy cream
• juice of 1 lemon

Wash and dry the fish, and cut into 2-inch/5-cm. pieces. Clean the mussels. Strip the stems from the sorrel, wash the leaves thoroughly, and drain them. Melt the butter in a large saucepan over medium heat. Add the sorrel, cover, and cook until the greens are wilted, 2 to 3 minutes. Remove the lid and continue cooking until all the moisture has evaporated. Sorrel will have dissolved to a purée; arugula will need to be chopped. Make the croûtes and set them aside.

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.