Shred the cabbage into thin strips.
Slice the onion thinly.
Heat cooking oil in a pan and stir-fry the onion and cabbage.
When the vegetables soften, add the tuna and continue to stir-fry.
Add gochujang, soy sauce, sugar, and minced garlic. Mix well and stir-fry.
Spoon the stir-fried mixture over rice to serve.