Slice cucumber and onion thinly.
Sprinkle salt over the cucumber and let it sit for a while.
Drain the oil from the canned tuna and prepare it.
Make the sauce by mixing gochujang (Korean chili paste), minced garlic, soy sauce, sugar, sesame oil, and vinegar.
Add cucumber, onion, tuna, and the sauce to the rice, then mix well.
Sprinkle sesame seeds on top to finish.