Shred the cabbage.
Slice the onion thinly.
Heat oil in a pan and sauté the onion.
When the onion turns translucent, add the cabbage and continue sautéing.
Once the cabbage softens, add the tuna and stir-fry together.
Season with soy sauce and pepper.
Serve the stir-fried mixture over rice and sprinkle with sesame seeds.