Thinly slice the cabbage.
Thinly slice the onion.
Heat cooking oil in a pan and sauté the onion and cabbage.
Season with salt and pepper.
Drain the oil from the canned tuna using a strainer.
Place the tuna on top of the sautéed vegetables and stir-fry together.
Add soy sauce, sugar, and sesame oil, and mix quickly.
Place the stir-fried mixture on top of the rice.
Garnish with seaweed flakes or green onions as desired.