Toss with the sesame oil. Add ¼ cup vinegar. Add more vinegar to task.
Add sesame seeds and red pepper flakes (if using).
Adjust seasoning.
Place in the fridge for an hour for the flavors to meld or serve immediately.
Notes
Your vinegar choices vary widely. I have used a simple rice vinegar (usually not sweet), mirin, or Ume. The latter is a red plum vinegar that is quite tart and salty and one of my favorite Asian vinegars.I prefer eating cucumbers with the peel since the peel has so many nutrients. However sometimes the peel on a particular cucumber is bitter, and should be removed. Also, some folks just don't like the peel. Your choice!