Finely chop onion, green chili pepper, and green onion.
Drain the oil from the canned tuna using a strainer.
In a bowl, combine the tuna, chopped vegetables, gochujang (Korean red chili paste), doenjang (soybean paste), minced garlic, sesame oil, and sesame seeds. Mix well.
Serve the completed tuna ssamjang on top of lettuce or perilla leaves.