Finely chop the cabbage.
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 together.
Once the vegetables are cooked, add the tuna and stir-fry.
Season with soy sauce and black pepper.
Place the stir-fried mixture on top of rice.