Shred the cabbage thinly.
Slice the onion thinly.
Drain the oil from the tuna using a strainer.
Add olive oil to a pan and sauté the onion.
When the onion turns translucent, add the cabbage and stir-fry.
Once the cabbage softens, add the tuna and stir-fry together.
Season with soy sauce and black pepper.
Serve the stir-fried mixture over rice to finish.