Shred the cabbage and finely chop the onion.
Drain the oil from the tuna can.
Heat cooking oil in a pan and stir-fry onion and cabbage.
When the vegetables are softened, add tuna and stir-fry together.
Season with soy sauce, sugar, and pepper.
Place the stir-fried mixture over rice.
Top with seaweed flakes and green onion as desired.