Wash the cabbage thoroughly and slice it into thin strips.
Slice the onion thinly.
Add olive oil to a pan and sauté the onions.
Add the cabbage and cook until softened.
Drain the oil from the tuna using a strainer.
Add the tuna to the sautéed vegetables and season with soy sauce and pepper.
Serve the stir-fried mixture over rice.
Finish by sprinkling seaweed flakes or green onions as desired.