Unless your rice cakes are soft already, soak them in warm water for 10 mins.
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.