Shred the cabbage thinly.
Finely chop the onion.
Heat cooking oil in a pan and sauté the onion first.
When the onion becomes translucent, add the cabbage and stir-fry.
Add tuna and mix well while stir-frying.
Season with soy sauce, pepper, and salt.
Place the stir-fried ingredients over rice.
Top with mayonnaise or seaweed flakes as desired.