Shred the cabbage and prepare it.
Slice the onion thinly.
Heat olive oil in a pan and sauté the onions.
When the onions become translucent, add the cabbage and stir-fry until slightly wilted.
Drain the oil from the canned tuna and place the tuna on top of the sautéed vegetables.
Season with soy sauce and pepper, and stir well while cooking.
Serve the sautéed cabbage and tuna over rice to finish.