What the hell is a chikuwa?
"Chikuwa is a type of processed seafood product that always comes in a tube shape, and is sometimes called a Japanese fish tube. This fishcake has a rubbery texture and a salty, fishy flavor. ... It can be eaten as a snack on its own or used in various soup and stir-fry recipes. It is also a popular edition to bento boxes, square Japanese lunch boxes that often contain rice, vegetables, and fish. "
Basically, just like in the anime, Chikuwa is Asuka's favourite snack.