Bring the 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 it 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 the veal stock, if using, and salt to taste. Add the fish and fish dumplings, bring to a boil, reduce the heat, and simmer for 7 to 8 minutes, 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.