Place all the fish bones and heads in a stockpot with 8 cups of water, the onions and a pinch of salt. Simmer, uncovered for 30 minutes. Place the mussels in a large saucepan with just the water that clings to them after washing. Cook over a low heat for 3-5 minutes, until opened. Strain the mussels, reserving any cooking liquid. In an ovenproof casserole or Dutch oven melt the butter and sautÄ the celery root, leeks, and carrots until softened, about 5 minutes. Cover and continue cooking for 15 minutes over a low heat, stirring occasionally. Add the fish fillets, fish stock and reserved mussel cooking liquid. In a small bowl whisk together the egg yolks, flour, crÅme fraöche, and 1 tablespoon lemon juice. Pour mixture into casserole. Season with nutmeg, salt and pepper. Bring to a boil, stirring. Reduce heat and simmer for 20 minutes. Just before serving, add the mussels. Serve in soup plates with grilled country bread and butter on the side.
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3 lb mixed white fish (John Dory, sea bream, whiting, conger eel, searobin), cut into fillets, heads and bones reserved
