Wash the cabbage thoroughly and slice it thinly.
Finely chop the onion.
Heat a pan, add some cooking oil, and saute the onions.
Add the cabbage and stir-fry together.
Drain the oil from the canned tuna and add the tuna to the vegetables.
Season with soy sauce, sugar, and pepper.
Place the stir-fried ingredients over rice and sprinkle with sesame seeds to finish.