Datemaki is a kind of egg dish. It is also called Datemaki-tamago. In Nagasaki it is also called Castella kamaboko.
Minced meat of whitefish or shrimp, with egg added to it, is mashed and mixed well, seasoned with mirin (sweet cooking rice wine) and sugar, and baked. It is rolled in makisu ("sushi mat", bamboo mat used in food preparation) while it is hot in order to arrange its shape. When you prepare it at home, instead of using minced fish, boiled fish-paste products called "hanpen", which is easier to acquire and made from the same raw material, can be used, but anyhow datemaki is essential to "osechi" Japanese special dishes prepared for the New Year.
Datemaki is in many aspects similar to Castella in cooking method, and resembles Atsuyakitamago (a thick omelet) in that they are both made from minced fish. It is believed that cooking techniques for European sweets were applied, from the fact that an oven is inevitable for making datemaki which tastes like sponge cake.
There are various theories as to the origin of the name "datemaki":It is called datemaki - because Masamune DATE liked it; - based on the word "datemono"which means classy, sophisticated costumes, because it is more gorgeous both in taste and in appearance than a normal omelet; or - because it looks like datemaki, a kind of slim sash used for females' wafuku (Japanese traditional clothes).