Shred the cabbage.
Slice the onion thinly.
Heat cooking oil in a pan and sauté the onions first.
When the onions turn translucent, add the cabbage and stir-fry together.
Once the cabbage softens, add the canned tuna after draining the oil.
Season with soy sauce, salt, and pepper.
Serve the sautéed mixture over rice.