Wash the cabbage and slice it thinly.
Slice the onion thinly.
Add cooking oil to a pan and stir-fry the cabbage and onion.
When the vegetables are wilted, add the tuna and stir-fry together.
Season with soy sauce, sugar, and pepper.
Place the stir-fried ingredients on top of the rice.
If you like, top with a fried egg to finish.