Slice the cabbage thinly.
Thinly slice the onion.
Heat cooking oil in a pan and sauté the onion.
When the onion becomes halfway translucent, add cabbage and stir-fry.
Drain the oil from the tuna, add it to the pan and stir-fry together.
Season with soy sauce, sugar, and pepper.
Place the stir-fried ingredients on top of rice.
Finish with mayonnaise or seaweed flakes, as desired.