Shred the cabbage thinly and prepare.
Slice the onion thinly.
Drain the oil from the canned tuna and set aside.
Heat a pan, add cooking oil, and stir-fry onion and cabbage.
Once the vegetables are cooked, add the tuna and stir-fry together.
Season with soy sauce, sugar, and pepper.
Place the stir-fried ingredients on top of rice.
Garnish with chopped green onions as desired to finish.