Wash the cabbage thoroughly and slice it thinly.
Slice the onion thinly.
Heat cooking oil in a pan and sauté the onion.
Add the cabbage and stir-fry until wilted.
Drain the oil from the tuna using a sieve and add it to the pan, stir-frying together.
Season with soy sauce and pepper.
Serve the stir-fried mixture over rice to finish.