Slice onion and cabbage into thin strips.
Heat cooking oil in a frying pan and sauté the onion.
When the onion becomes translucent, add the cabbage and stir-fry together.
Drain the oil from the canned tuna and add the tuna to the pan.
Season with soy sauce and sugar to taste.
Place the stir-fried ingredients over rice.
If desired, finish with seaweed flakes and sesame seeds.