Wash the cabbage thoroughly and shred it thinly.
Slice the onion thinly.
Heat olive oil in a pan and sauté the onion.
Add the cabbage and stir-fry until it softens.
Drain the oil from the canned tuna and add the tuna to the pan. Stir-fry together.
Season with salt and pepper to taste.
Serve the stir-fried mixture over rice.