Wash the cabbage thoroughly and slice it thinly.
Slice the onion thinly.
Heat olive oil in a pan and sauté the onion.
When the onion turns translucent, add the cabbage and stir-fry.
Once the cabbage softens, add the tuna (drained of oil) and stir-fry together.
Season with soy sauce and pepper.
Spoon the stir-fried mixture over rice to serve.