Wash the napa cabbage and duck thoroughly, then cut them into bite-sized pieces.
Slice the onion, green onion, and garlic.
Season the duck with salt, pepper, and minced garlic.
Line a pot with napa cabbage, then add duck, onion, and green onion on top.
Mix soy sauce, red pepper flakes, sesame oil, and sugar to make a seasoning sauce.
Evenly drizzle the seasoning sauce over the ingredients.
Cover with a lid and steam on medium heat for 20 minutes, then mix the ingredients.
Steam for another 10 minutes until everything is fully cooked.