Cut the napa cabbage into bite-sized pieces.
Thinly slice the onion.
Drain the oil from the tuna using a strainer.
In a bowl, add napa cabbage, onion, and tuna.
Add gochugaru (Korean red pepper flakes), minced garlic, sesame oil, salt, sugar, and sesame seeds, then mix well.
Transfer to a plate and sprinkle with extra sesame seeds to finish.