Wash the napa cabbage thoroughly and cut into pieces about 4-5cm long.
Thinly slice the onion and green onion.
In a bowl, mix soy sauce, red pepper powder, minced garlic, sugar, and sesame oil to make the seasoning sauce.
Layer the onion and green onion on the bottom of a pot, then place the napa cabbage on top.
Evenly spread the seasoning sauce over the napa cabbage.
Cover with a lid and steam over medium-low heat for 10-15 minutes.
When the cabbage is tender, transfer to a plate and serve.