Finely shred the cabbage.
Slice the onion thinly.
Add cooking oil to a pan and stir-fry the onion and cabbage.
Once the cabbage and onion have softened, add the tuna and stir-fry together.
Season with soy sauce, sugar, and pepper.
Place the stir-fried mixture over rice.
Optionally, top with seaweed flakes or a fried egg as desired.