Wash the cabbage thoroughly and slice it thinly.
Slice the onion.
Heat olive oil in a pan and sauté the onion first.
When the onion turns translucent, add the cabbage and stir-fry together.
Once the cabbage softens, add tuna (drained of oil) and stir-fry together.
Season with salt and pepper.
Serve the stir-fried ingredients over rice to finish.