Wash the cabbage thoroughly and cut it into bite-sized pieces.
Slice the onion thinly.
Heat cooking oil in a pan and sauté the onion and cabbage.
Once the cabbage softens, add the drained tuna and stir-fry together.
Season with soy sauce and pepper.
Place the sautéed mixture over rice.
If desired, sprinkle with roasted seaweed flakes and drizzle with sesame oil to finish.