Prepare the cabbage by slicing it thinly.
Slice the onion thinly.
Add olive oil to a pan and sauté the onions.
Add the cabbage and sauté together.
Drain the oil from the canned tuna and add tuna to the pan, stir-frying together.
Season with soy sauce and black pepper.
Serve the sautéed ingredients over rice to finish.