Wash the cabbage and cut it into bite-sized pieces.
Thinly slice the onion.
Heat some cooking oil in a pan and stir-fry the onion and cabbage.
When the cabbage softens, add the tuna and mix well.
Season with soy sauce and pepper.
Place the stir-fried mixture over the rice.
Optionally, top with mayonnaise and seaweed flakes to taste.