Shred the cabbage.
Slice the onion thinly.
Heat cooking oil in a pan and sauté the onion and cabbage.
Drain the oil from the canned tuna and add it to the pan. Stir-fry together.
Season with soy sauce, sugar, and pepper.
Place the stir-fried mixture on top of the rice.
Finish by sprinkling sesame seeds and green onions on top.