Shred the cabbage.
Slice the onion thinly.
Heat cooking oil in a pan and sauté the onions.
Add the cabbage and stir-fry until it softens.
Drain the oil from the tuna can and add the tuna to the pan. Stir-fry together.
Season with soy sauce and pepper to taste.
Place the stir-fried mixture on top of the rice.