Cut the rice cakes, fish cakes, and onion into bite-sized pieces.
In a pot, add water, gochujang (Korean chili paste), gochugaru (chili powder), soy sauce, and sugar, then bring to a boil.
Once the sauce is boiling, add the rice cakes, fish cakes, and onion.
Simmer over medium heat, stirring until all ingredients are cooked through.
Add chopped green onion, bring to a boil once more, then turn off the heat.