Soak frozen rice cakes in warm water for 15 minutes if needed.
Mix gochujang, gochugaru, garlic, ketchup, sugar, and soy sauce in a bowl.
Bring vegetable stock and chopped spring onion whites to a boil in a pot.
Add rice cakes and cook for 2 minutes.
Pour in the sauce and simmer until thick, about 5–10 minutes.
Add green onions and simmer a few more minutes.
Top with sesame oil, seeds, and chopped green onions.
