Soak tteokguk rice cakes in cold water for about 10 minutes.
Slice fish cake, onion, and green onion into bite-sized pieces.
Add water, gochujang, gochugaru, soy sauce, and sugar to a pan and bring to a boil.
Once the sauce boils, add the soaked rice cakes and cook for 3 minutes.
Add fish cake, onion, and green onion, and cook for another 3-5 minutes.
When the rice cakes and fish cake are soft, finish with sesame oil and roasted sesame seeds.