Place the tuna in a strainer to remove the oil.
In a bowl, mix the tuna, chopped onion, chopped Cheongyang pepper, and minced garlic.
Add gochujang (red chili paste), doenjang (soybean paste), oligosaccharide syrup, sesame oil, and ground sesame seeds, then mix well.
Transfer the finished tuna ssamjang to a plate.