Drain the oil from the canned tuna and set aside.
Finely chop the onion and green onion.
In a bowl, combine tuna, chopped onion, mayonnaise, and pepper. Mix well.
Cook the noodles in boiling water, rinse with cold water, and drain thoroughly.
Make the sauce by mixing gochujang (Korean chili paste), soy sauce, sugar, vinegar, and sesame oil.
Place the noodles in a bowl and mix with the sauce.
Top with the tuna mayo mixture and sprinkle with seaweed flakes to finish.