Slice the cabbage into thin strips and set aside.
Slice the onion thinly.
Drain the oil from the canned tuna and set aside.
Heat olive oil in a pan and sauté the onion.
When the onion becomes translucent, add the cabbage and stir-fry.
Once the cabbage softens, add the tuna and season with salt and pepper.
Place the stir-fried mixture over rice to finish.