If you live in a community with mostly Ashkenazi Jews, at holiday time you are likely to see a lot of traditional Ashkenazi foods. Gefilte fish is perhaps the most mysterious to those who didn’t grow up eating it.
“Gefilte” (geh-FILL-teh) is not a species of fish. The word is Yiddish for “filled.” Gefilte fish is a fish loaf: boned, minced white fish mixed with matzo crumbs, chopped onions and root vegetables, eggs and seasonings. (Think “meat loaf” only with fish.) Then it is formed into balls and the balls are poached either in water or more usually in fish stock.
Gefilte fish is served cold, often with horseradish and a carrot or egg garnish.
As with many Ashkenazi foods, gefilte fish developed in response to regional food availability and ritual requirements. Fish is an especially flexible menu item in a kosher diet, because it is parve, that is, it can be eaten with either flesh or dairy. Moreover, mixing the fish with crumbs of matzo or bread crumbs stretches the expensive protein.
However, fish is tricky on Shabbat, since boning it is viewed by many traditional sources to be a violation of the Sabbath. (Sorting or picking one thing out from another is called borer and is one of the 39 forbidden classes of activity.) Therefore the bones must be removed before Shabbat! Gefilte fish work nicely for this, since the boning happens before Shabbat, and the dish is eaten cold – one less thing to keep hot for the Sabbath meal.
Advice for Beginners: Horseradish helps!