Wash the cabbage thoroughly and shred it.
Slice the onion thinly.
Drain the oil from the tuna using a strainer.
Heat some cooking oil in a pan and stir-fry the cabbage and onion.
When the cabbage and onion soften, add the tuna and stir-fry together.
Season with soy sauce, salt, and pepper to taste.
Serve the stir-fried mixture over rice.
If desired, add mayonnaise or parsley on top to finish.