Soak the rice cakes in cold water for 10 minutes.
Cut the fish cake, onion, and green onion into bite-sized pieces.
Add a bit of olive oil to a pan and stir-fry the onion and green onion.
Add the fish cake and rice cakes, then stir-fry for another 2 minutes.
Add water, gochujang (red chili paste), chili powder, minced garlic, and erythritol, then bring to a boil.
Reduce to low heat and simmer for 5–7 minutes.
Sprinkle sesame seeds over the finished tteokbokki.