Thinly slice the cabbage.
Thinly slice the onion.
Drain the oil from the canned tuna using a strainer.
Heat cooking oil in a pan and sauté the onions.
When the onions become translucent, add the cabbage and stir-fry.
When the cabbage softens, add the tuna and stir-fry together.
Season with soy sauce and black pepper to taste.
Serve the sautéed mixture over rice to finish.