Rice cakes
Korean tteok: steamed and pounded rice dough, usually shaped into finger-thick sticks or oval slices. Firmly chewy and mild, not to be confused with puffed rice cakes.
Tteok is a dough made from rice flour that is steamed and then pounded or kneaded until dense and elastic. For braises and soups it is shaped into long sticks cut into pieces, or sliced on the diagonal into thin ovals. It has little flavour of its own and takes on the sauce or broth around it. What matters is the texture: soft on the outside, chewy and springy within. Fresh tteok hardens quickly in the fridge but softens again when cooked. Older or chilled pieces are soaked in cold water for a while beforehand. It is sold in Asian grocers chilled, frozen or vacuum-packed, sometimes dried, and is also available by mail order.
Substitute:
There is no good substitute. Japanese mochi is made from glutinous rice, turns soft when cooked and melts away. Gnocchi or short thick pasta will carry the sauce but lack the chewy, springy bite these dishes are built around.