Shred the cabbage.
Slice the onion thinly.
Heat cooking oil in a pan and stir-fry the onion.
When the onion turns translucent, add the cabbage and stir-fry.
Drain the oil from the tuna, add it to the pan, and stir-fry together.
Season with soy sauce and pepper.
Place the stir-fried mixture over rice to complete the rice bowl.