1) For the gochuchang paste: combine all ingredients in a small bowl. Mix well.
2) Whisk together the vinegar, soya, dashida, sugar, gochuchang, and sesame oil in a large bowl. Add the cucumber and onion and stir until combined. Sprinkle with sesame seeds and let marinate at least 1 hour before eating.
Note: Gochuchang (GOH-choo-jang) paste: spicy red pepper paste sold either in glass jars or plastic containers that can be purchased at any Korean or Asian food market.