Bring fish stock to a boil in a large pot. Add the vegetables, reduce the heat, and let simmer for 5 minutes.

If you are using the flour, whisk in flour together with the cream in a small bowl, add the cream to the soup, and bring to a boil. Add the sugar and vinegar to taste; the soup should have a subtle sweet-and-sour flavor. Add salt to taste. Add the fish and fish dumplings, bring to a boi, reduce the heat, and simmer for 7 to 8 minutees, until the fish is just cooked.

In the meantime, whisk together the egg yolks and sour cream in a small bowl. Pour the soup into bowls, dividing the fish and dumplings evenly. Gently stir 2 to 3 tablespoons of the sour cream mixture into each bowl. Sprinkle with chives, if desired, and serve.

Fish Dumplings

Run the fish through a meat grinder twice: it should be very fine and smooth. (You can also use a food processor, although it can be more diffiicult to get a smooth consistency that way.)

In a large bowl, lightly beat the eggs. Add the cornstarch, salt, pepper, and fish and mix well. gently fold the cream into the fish mixture, trying not to deflate cream.

With a spoon, form the dumpling mixture into small balls.