Wash and shred the cabbage.
Slice the onion thinly.
Heat cooking oil in a pan and stir-fry the onion and cabbage.
Drain the oil from the canned tuna and add the tuna to the pan.
Season with salt and pepper to taste.
Place the stir-fried ingredients on top of the rice.
Optionally, add seaweed flakes or mayonnaise as desired.