Wash the cabbage thoroughly and slice it thinly.
Slice the onion thinly.
Heat cooking oil in a pan and sauté the onion and cabbage.
Drain the oil from the tuna and add it to the pan, stir-frying together.
Season with soy sauce, sugar, and black pepper.
Place the sautéed mixture over rice.
Add your desired toppings (seaweed flakes, mayonnaise, etc.).