Shred the cabbage and thinly slice the onion.
Drain the oil from the canned tuna using a strainer.
Heat cooking oil in a pan and stir-fry the onion and cabbage.
Once the vegetables have softened, add the tuna and continue to stir-fry.
Season with soy sauce, sugar, and pepper.
Serve the stir-fried mixture over rice.
Garnish with seaweed or sesame seeds as desired.