Cut the rice cakes, fish cakes, onion, and green onion into bite-sized pieces.
Pour water into a pan and add gochujang (Korean red chili paste), gochugaru (red chili powder), sugar, soy sauce, and minced garlic to make the sauce.
When the sauce starts to boil, add the rice cakes, fish cakes, and onion.
Simmer over medium heat for about 10 minutes. Once the rice cakes become soft, add the green onion.
Once the sauce thickens and all ingredients are cooked through, it's ready to serve.