Cut the cabbage into bite-sized pieces.
Thinly slice the onion.
Heat cooking oil in a pan and sauté the onion.
When the onion becomes slightly translucent, add the cabbage and stir-fry together.
Once the vegetables are cooked, add the tuna and stir-fry together.
Season with soy sauce, sugar, and pepper.
Place the stir-fried ingredients over rice.
Sprinkle sesame seeds and seaweed flakes on top to finish, if desired.