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1. Combine the onions, carrot, parsnip, celery root, parsley sprigs, allspice, bay leaves and 1 tablespoon of the salt in a 3- to 4-quart enameled or stainless - steel saucepan.
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2. Pour in ¼ cup of the vinegar and the 6 cups of water, and bring to a boil over high heat. Reduce the heat to low and simmer partially covered for 45 minutes.
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3. Strain the liquid through a large fine sieve into a fish poacher or large, deep roasting pan, pressing down hard on the vegetables with the back of a spoon to extract all their juices before discarding them. Ladle 1/3 cup of the strained liquid into a cup or bowl and set it aside.
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4. Wash the fish inside and out under cold running water. Without drying it, wrap the fish in a long, double-thick piece of dampened cheesecloth, leaving at least 6 inches of cloth at each end to serve as handles for lifting the fish.
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5. Twist the ends of the cloth close to the fish and tie them with string. Then place the fish on the rack of the poacher or roasting pan and lower it into the poacher or pan.
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6. If you are using a roasting pan, tie the ends of the cheesecloth to the handles of the pan. If necessary, add enough cold water to cover the fish by about 2 inches.
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7. Cover the pan and bring to a boil over moderate heat; immediately reduce the heat to low and simmer slowly for about 20 minutes, or until the fish feels firm when prodded gently with a finger.
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8. Using the ends of the cheesecloth as handles, lift the fish from the pan and lay it on a large cutting board or platter. Open the cheesecloth and skin the fish with a small, sharp knife by making a cut in the skin at the base of the tail and gently pulling off the skin in strips from tail to gill.
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9. Pick off and discard any bits of skin clinging to the cheesecloth. Holding both ends of the cheesecloth, carefully lift the fish and turn it over onto an ovenproof platter or shallow baking-serving dish.
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10. Peel off the skin on the upturned side and drape the fish loosely with aluminum foil. Set aside. Preheat the broiler to its highest possible setting.
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11. Place the potato balls in a small saucepan and add enough cold water to cover them completely. Bring to a boil over high heat and cook briskly uncovered for about 12 minutes, or until the potatoes are tender and show no resistance when pierced deeply with the point of a small skewer or knife.
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12. Drain the potatoes and return them to the pan. Sliding the pan back and forth constantly over low heat, cook for 1 to 2 minutes, until the potatoes are dry. Cover the pan with a lid or foil to keep them warm.
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13. In a heavy 8- to 10-inch skillet, melt 3 tablespoons of butter over moderate heat. When the foam begins to subside, add the flour and mix thoroughly with a wire whisk.
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14. Whisk in the reserved 1/3 cup of strained liquid and stir until the sauce comes to a boil, thickens heavily and is smooth.
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15. Reduce the heat to low and add the sour cream, sugar, horseradish, the remaining 3 tablespoons of vinegar and the remaining ½ teaspoon of salt. Whisking frequently, simmer for 3 or 4 minutes.
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16. In a small bowl, beat the egg yolk lightly with a fork and stir in 2 tablespoons of the simmering sauce. Stirring constantly with the whisk, pour the egg mixture back into the main sauce in a slow, thin stream. Immediately remove the sauce from the heat and taste for seasoning.
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17. Pour the sauce over the fish and spread it smoothly with a spatula to mask the fish completely. Scatter the 2 tablespoons of butter bits on top and slide the fish under the broiler, 3 or 4 inches from the heat. Broil for 2 to 3 minutes, until the sauce is delicately browned, watching it constantly to make sure it does not burn.
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18. Arrange the potato balls attractively around the fish, sprinkle them with parsley, and serve at once. To debone the fish for serving, divide the top layer into 2 portions with a fish server without cutting through the spine.
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19. Leave the head and tail intact. Lift the portions with the server and a fork and arrange them attractively on serving plates. Then gently remove the backbone in one piece, discard it, divide the bottom layer of fish into individual portions as before and spoon the sauce over each portion.
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