Cut the rice cakes and fish cakes into bite-size pieces.
Slice the onion thinly.
In a pan, add water, gochujang (Korean red chili paste), red pepper powder, soy sauce, and sugar, then bring to a boil.
Add the rice cakes, fish cakes, and onions and simmer over medium heat.
When the rice cakes become soft, add green onions and boil once more.