Soak the rice cakes in warm water for 10 minutes. (If you are using fresh rice cakes that are already tender, feel free to skip this step. However, if you are using packaged rice cakes from the fridge, it is recommended to soak them to enhance their moisture level before proceeding with the recipe.)
Boil the soup stock in a shallow pot over medium high heat and dissolve the tteokbokki sauce by stirring it with a spatula. Once the seasoned stock is boiling, add the rice cakes, fish cakes and onion. Boil them a further 3 to 5 mins until the rice cakes are fully cooked. Then, to thicken the sauce and to deepen the flavor, simmer it over low heat for a further 2 to 4 mins.
Add the sesame oil, sesame seeds, and green onion then quickly stir. Serve warm.