Wash the napa cabbage thoroughly and cut the large leaves in half.
Finely chop the onion, green onion, and garlic.
Lay the napa cabbage evenly in the steamer.
Sprinkle the chopped onion, green onion, and garlic evenly over the cabbage.
Mix soy sauce, red pepper flakes, sesame oil, and sugar to make the sauce.
Drizzle the sauce evenly over the cabbage.
Pour water into the steamer and steam for 15-20 minutes.
Transfer the finished steamed napa cabbage to a plate.