Cut the rice cakes and fish cakes into bite-sized pieces.
Add water, gochujang (Korean chili paste), gochugaru (chili powder), sugar, and soy sauce to a pan and bring to a boil.
Add the rice cakes and simmer over medium heat for 5 minutes.
Add fish cakes, onion, and green onion, and cook for another 5 minutes.
When the sauce thickens, it's ready to serve.