Wikipedia
A Fischbrötchen (lit. fish bun) is a sandwich made with fish and other components like fresh white or dried onions, pickles, remoulade, creamy horseradish sauce, ketchup, or cocktail sauce. It is commonly eaten in Northern Germany, due to the region's proximity to the North Sea and Baltic Sea.
A common preparation is made with bismarck herring or soused herring. Other varieties use Brathering, rollmops, European sprat, salmon, smoked Atlantic mackerel, other fried fish varieties (e.g. fish burgers). Prawns are sometimes used, as are various other species of food fish. Fischbrötchen are commonly served at fast food stands or take-out restaurants.
The Hanover Fair was initially colloquially known as the Fischbrötchen fair due to the fish buns served there as a snack.