Shred the cabbage thinly.
Slice the onion thinly.
Heat olive oil in a pan and sauté the onion.
When the onion becomes translucent, add the cabbage and stir-fry.
Season with salt and pepper.
Drain the oil from the canned tuna and add to the pan; stir-fry together.
Place the stir-fried ingredients over the rice.