Shred the cabbage into thin strips.
Slice the onion thinly.
Heat cooking oil in a pan and sauté the onions.
When the onions turn translucent, add the cabbage and stir-fry.
Once the cabbage has softened, add the tuna and mix well while stir-frying.
Season with soy sauce, sugar, and pepper to taste.
Place the stir-fried cabbage and tuna over rice.
Top with crushed seaweed or sesame seeds if desired.