Slice cucumber and onion thinly.
Sprinkle salt over the sliced cucumbers and let them sit for about 10 minutes.
Squeeze out the excess water from the salted cucumbers.
In a bowl, mix the cucumber, onion, gochugaru (Korean chili flakes), vinegar, sugar, salt, and minced garlic well.
Add sesame oil and roasted sesame seeds, then toss once more.