Shred the cabbage.
Slice the onion thinly.
Add oil to a pan and sauté the onion.
When the onion becomes translucent, add the cabbage and stir-fry.
Drain the oil from the tuna using a strainer.
Place the tuna on top of the sautéed vegetables, add soy sauce and pepper, and stir-fry.
Spoon the stir-fried mixture over rice.