Wash the cucumber thoroughly and slice it thinly.
Slice the onion thinly and soak in cold water to remove the spicy taste.
Place the cucumber and onion in a bowl and sprinkle with a bit of salt to lightly season.
Remove excess moisture from the salted cucumber and onion.
Add vinegar, sugar, salt, and sesame oil and gently mix.
Sprinkle sesame seeds to finish.