Wash the cucumbers thoroughly.
Slice the cucumbers into thin rounds or half-moons.
Julienne the carrot and set it aside.
Thinly slice the red onion.
In a small bowl, whisk together sesame oil, rice vinegar, soy sauce, and sugar.
Combine cucumbers, carrots, and onions in a large mixing bowl.
Pour the dressing over the vegetables.
Toss the salad gently to coat all ingredients evenly.
Sprinkle sesame seeds and red chili flakes if desired.
Garnish with fresh cilantro before serving.