Wash the cabbage and slice it thinly.
Slice the onion thinly.
Heat some oil in a pan and sauté the onions.
Add the cabbage and stir-fry together.
Drain the oil from the tuna and add it to the pan, stir-frying.
Season with soy sauce, sugar, and pepper.
Place the stir-fried mixture over rice.
Top with seaweed flakes or sesame seeds as desired.