Shred the cabbage and thinly slice the onion.
Add cooking oil to a pan and stir-fry the onion and cabbage.
Once the cabbage softens, add the tuna and stir-fry together.
Season with soy sauce, gochujang (Korean chili paste), sugar, and pepper.
Serve the stir-fried mixture over rice and sprinkle seaweed flakes on top if desired.