FISH STOCK: To prepare fish stock, which will be the base of the soup, combine the ingredients listed under that heading (above) in a 4 to 6 litre stock pot. Bring to a boil, partially cover the pot, turn the heat low and simmer for 30 to 40 minutes. Strain the stock through a fine sieve into a large bowl, pressing down hard on the vegetables and fish trimmings with the back of a spoon to extract their juices before discarding them. Wash the pot and return the strained stock to it. Reduce the stock to about 6 cups by boiling it rapidly, uncovered, for about 20 minutes. Restrain through a fine sieve or through a double thickness of cheesecloth lining a regular sieve. SOUP: Again return the stock to the pot. Add the carrots, parsnips and fish. As soon as the soup reaches the boil, lower the heat and simmer uncovered for about 10 minutes. Add the leeks and simmer 2 or 3 minutes longer. Remove from heat, lift out the fish with a slotted spoon and set aside on a platter. In a small bowl, beat egg yolks with a wire whisk; then beat in about half cup of hot soup, 1 tablespoon at a time. Pour this back into the soup in a thin stream, beating continuously with a wire whisk. With a fork, separate the fish into flakes and add it to the soup. Season with sale and pepper and reheat, but do not let the soup boil. To serve, ladle the soup into individual bowls and spinkle with chopped parsley. If you like, garnish each serving with 1 tablespoon sour cream.