Slice the onion and kimchi into bite-sized pieces.
Heat cooking oil in a pan and sauté the onion.
Add the kimchi and stir-fry.
Drain the oil from the canned tuna and add it to the pan.
Add gochujang (Korean chili paste), soy sauce, and sugar, then stir-fry.
Drizzle with sesame oil and sprinkle with sesame seeds.