Thinly slice the cabbage.
Slice the onion thinly.
Add olive oil to a pan and sauté the onion.
When the onion becomes translucent, add the cabbage and stir-fry.
Drain the oil from the canned tuna and add it to the pan. Stir-fry together.
Season with soy sauce and pepper.
Serve the stir-fried ingredients over rice.