Slice the cucumbers and onion as thinly as possible.
In a large bowl, whisk together the vinegar, olive oil, sugar (if using), salt, and black pepper.
Add the cucumbers, onion, and chopped herbs.
Toss until everything is evenly coated.
Refrigerate for 15–30 minutes before serving to let the flavors develop.
Toss once more and serve chilled.
Optional Add-Ins
Crumbled feta cheese
Cherry tomatoes, halved
Avocado, diced
Thinly sliced radishes
Toasted sesame seeds
Chili flakes for a little heat
Tips
For an extra-crisp salad, sprinkle the cucumber slices with a little salt and let them sit for 15 minutes. Pat them dry before mixing with the dressing.
English or Persian cucumbers work best because they have thinner skins and fewer seeds.
This salad is best enjoyed the same day but can be refrigerated for up to 24 hours.