Unless your rice cakes are soft already, soak them in warm water for 10 mins.
Boil the yuksu/dashi broth 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.
Thicken the sauce and deepen the flavor by simmering over a low heat for a further 2 to 4 mins.
Add the sesame oil, sesame seeds, and green onion then quickly stir. Serve warm.