Thinly slice the cabbage.
Slice the onion.
Heat cooking oil in a pan and sauté the onion.
Once the onion becomes translucent, add the cabbage and stir-fry together.
When the vegetables are cooked, add the tuna and mix well.
Season with soy sauce, sugar, and pepper.
Place the stir-fried mixture over rice.
Top with seaweed flakes or mayonnaise if desired for a finishing touch.