Shred the cabbage into thin strips.
Slice the onion thinly.
Heat olive oil in a pan and sauté the onion.
Add the cabbage and stir-fry until it softens.
Drain the oil from the canned tuna and add it to the pan, stir-frying together.
Season with soy sauce and black pepper to taste.
Serve the stir-fried mixture on top of rice to finish.