Shred the cabbage.
Thinly slice the onion.
Heat olive oil in a pan and sauté the onion.
When the onion becomes translucent, add the cabbage and stir-fry.
Drain the oil from the canned tuna, add the tuna to the pan, and stir-fry.
Season with soy sauce and pepper.
Serve the stir-fried mixture over rice.