Wash cucumbers. Peel if using regular cucumbers.
Slice cucumbers very thin with a knife or mandoline.
Thinly slice red onion. Separate the rings.
Place cucumbers and onions in a large bowl.
In a small bowl, whisk vinegar, water, sugar, salt, pepper, and dill until sugar dissolves.
Add olive oil if using. Whisk again.
Pour dressing over cucumbers and onions.
Toss gently to coat everything.
Cover and refrigerate for at least 1 hour.
Drain excess liquid before serving.
Toss once more and transfer to serving bowl.