In a large bowl, combine the sliced cucumber, red onion, and julienned carrot.
In a small bowl, whisk together the rice vinegar, sesame oil, soy sauce, and sugar until well combined.
Pour the dressing over the vegetables and toss to coat evenly.
Sprinkle the sesame seeds and red pepper flakes over the salad, then toss again to distribute.
Let the salad sit for about 5-10 minutes for the flavors to meld before serving.
Notes
This salad can be made ahead of time and stored in the refrigerator for up to a day. You can also add other ingredients like cilantro or sliced chili for extra flavor.