This soup was highly praised by Ivan the Terrible--who wasn't easy to please--and by Antoine Careme--who was even harder to please, at least gastronomically. As the oldest recorded Russian soup, ukha began life as any digestible food in a liquid, but by the 16th and 17th century lavish Russian courts, it was transformed into elaborate and diverse creations, served between pie courses. It was in the 18th century that it became associated exclusively with fish--and in the 19th century, under the influence of French cuisine, that it became the delicate creation that it is today. A light, clean first course; serve this Russian soup hot to 6 people.
Bring stock to a boil in a large saucepan. Lower in the fish fillets and reduce heat to low. Simmer for 3-4 minutes--until the fish is just opaque. Carefully lift the fish out and put into flat soup bowls. Pour hot stock on top, squeeze a little lime juice into each bowl, float a thin lime slice on top, and sprinkle with dill. Serve at once.