Wash cabbage thoroughly and shred it.
Finely chop the onion.
Heat olive oil in a pan and sauté the onion.
When the onion becomes translucent, add the cabbage and stir-fry.
Drain the oil from the tuna using a strainer, then add the tuna to the pan and stir-fry.
Season with salt and pepper.
Top steamed rice with the stir-fried ingredients to finish.