Whisk together the vinegar, soy, 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.
Gochuchang Paste (seasoned red pepper paste):
Combine all ingredients in a small bowl. Mix well.
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.