Wash the cucumber thoroughly and trim both ends.
Slice the cucumber thinly.
Sprinkle a little salt over the sliced cucumbers and let them sit for 5 minutes.
Rinse the salted cucumbers in cold water and drain off the excess water.
Thinly slice the onion.
Mix sugar, vinegar, soy sauce, minced garlic, sesame oil, and red pepper powder to make the dressing.
Add the dressing to the cucumber and onion, and mix well.
Garnish with sesame seeds and chili peppers, as desired.