Shred the cabbage.
Slice the onion thinly.
Heat oil in a pan and sauté the onion.
Add the cabbage, sprinkle a bit of salt, and stir-fry together.
Drain the oil from the canned tuna and add the tuna to the pan, stir-frying it.
Season with soy sauce and pepper.
Top steamed rice with the stir-fried cabbage and tuna mixture to finish.