Shred the cabbage thinly.
Slice the onion thinly.
Heat cooking oil in a pan and sauté the onion.
When the onion turns translucent, add the cabbage and stir-fry together.
Drain the oil from the canned tuna, add to the pan, and stir-fry.
Season with salt and pepper to taste.
Serve the stir-fried mixture over rice.