This is Anago, a perfectly prepared eel with a touch of Tsume, a sweet reduction of Anago, etc. Tsume is a sweet glaze used in old fashion Edo style sushi for cooked ingredients. The word Tsume comes from Nitsume or reduction. The Tsume sauce is made from slowly poaching the Eel with Sugar, Mirin and time, but it is all up the chef’s own idea. That makes the difference.