Cut the cabbage into bite-sized pieces.
Thinly slice the onion.
Drain the oil (or water) from the canned tuna using a strainer.
Place the cabbage on the bottom of a pot.
Layer the onion and then the tuna on top of the cabbage.
Season with salt and pepper.
Cover with a lid and steam over medium-low heat for 10-15 minutes.
After cooking, drizzle soy sauce or your preferred sauce to taste.