Shred the cabbage.
Slice the onion.
Heat some cooking oil in a pan and stir-fry the onion and cabbage.
Once the vegetables soften, add the tuna and stir-fry together.
Season with soy sauce and pepper to taste.
Spoon the stir-fried cabbage and tuna over rice.
Garnish with seaweed flakes or sesame seeds as desired.