1 pound fish bones from any saltwater fish except salmon, cut into pieces
1 carrot, sliced
1 shallot, sliced
1/2 onion, sliced
1/2 stalk celery, sliced
2 cups dry white wine
2 1/2 pounds fresh mussels in their shells
1/2 pound carrots, peeled
1/2 pound leeks, trimmed, thoroughly washed
1/2 pound celery, trimmed
1 cup dry white wine
1 tablespoon unsalted butter
1/2 teaspoon saffron threads
1/2 teaspoon dried thyme
2 cups heavy cream
Freshly ground black pepper
Juice of 1/2 a small lemon
To make the Fish Stock, first prepare a bouquet garni by wrapping the parsley, celery leaves, thyme, bay leaf and peppercorns inside a square of cheesecloth, tying it securely with kitchen string.
In a medium saucepan, heat the olive oil over medium heat. Add the fish bones, carrot, shallot, onion and celery and saute, stirring frequently, for 10 minutes. Add the wine and stir and scrape to deglaze the pan deposits. Add the bouquet garni and enough cold water to cover. Bring to a boil, using a large spoon to skim off foam as it rises to the surface. Reduce the heat and simmer for 25 minutes. Strain through a cheesecloth-lined fine-meshed strainer set over a heatproof bowl. You should have about 1 quart. Reserve 2 cups for the soup, refrigerating the rest in a covered container for another use.
For the Mussel Soup, thoroughly wash and scrub the mussels under cold running water, pulling off their beardlike strands and removing all sand. Discard any that seem overly light, meaning they're probably dead, or any that feel very heavy, meaning they're full of sand.
Coarsely chop half each of the carrots, leeks and celery, setting the remainder aside. Put the chopped vegetables in a large saucepan and add the mussels, 2 cups fish stock and wine. Bring to a boil over medium-high heat, cover and cook for about 5 minutes, until the shells have opened. Strain the contents of the pan through a cheesecloth-lined strainer set over a large heatproof bowl. Reserve the liquid. Put the mussels in a separate bowl, reserving them and discarding any unopened ones along with the other solids.
Cut the remaining vegetables into uniform 1/4-inch dice. In a 6-quart saucepan over low heat, melt the butter. Add the diced vegetables, saffron and thyme and saute gently for about 10 minutes.
Add the cream. Raise the heat to medium-high and boil, stirring frequently, until the cream reduces and thickens slightly, about 7 minutes. Stir in the strained cooking liquid, raise the heat and continue boiling until the mixture has reduced slightly, about 5 minutes more.
Meanwhile, shuck the cooked mussels, prying their shell halves apart and pulling out the meat. Reduce the heat to low and add the shucked mussels to the soup to reheat briefly. Season to taste with salt, black pepper, a dash of cayenne and lemon juice.