Prepare the cabbage by slicing it thinly.
Thinly slice the onion.
Add cooking oil to a pan and sauté the onions.
Once the onions turn translucent, add the cabbage and stir-fry.
Drain the oil from the tuna and add it to the pan.
Season with soy sauce, sugar, and pepper.
Place the stir-fried ingredients over steamed rice.