Soak the rice cakes in cold water for 10 minutes.
Cut the fish cake into bite-sized pieces.
Put water, gochujang, gochugaru, soy sauce, and sugar into a pot and bring to a boil.
Once the sauce boils, add the rice cakes, fish cake, and onion.
Slice the green onion and add it, then simmer over medium heat for 10 minutes.
Sprinkle black pepper at the end.