Wash the napa cabbage thoroughly and cut it into bite-sized pieces.
Finely chop the onion and green onion.
Drain the oil from the canned tuna using a strainer.
Heat cooking oil in a pan and sauté the onion and green onion.
Add the napa cabbage and stir-fry until it softens.
Add the tuna, then season with soy sauce and pepper.
Place the stir-fried mixture over rice.
Sprinkle with sesame seeds to finish.