Shred the cabbage.
Thinly slice the onion.
Add cooking oil to a pan and sauté the onion and cabbage.
When the cabbage softens slightly, add the tuna and stir-fry together.
Season with soy sauce, sugar, and pepper.
Place the sautéed mixture over rice.
Sprinkle mayonnaise or seaweed flakes on top according to your preference.