1. Gut, wash, and cut the fish into pieces. Wash the vegetables. Finely slice the leeks, celery, and onion. Sauté them in butter.
2. Season. Add the thyme, bay leaf, and parsley roots, tied together in a bouquet.
3. Pour in the fish stock and white wine. Let it simmer for 20 minutes over medium heat.
4. Remove the fish from the cooking liquid using a slotted spoon. Place it in a deep dish and keep it warm.
5. Remove the bouquet garni and check the seasoning. Thicken the cooking liquid with the egg yolks diluted in cream.
6. Serve the broth with toasted bread croutons at the same time as the fish, but in separate bowls.
7. Sprinkle the fish with chopped parsley and garnish with lemon slices.