Wash cucumbers thoroughly and pat dry. If using regular cucumbers with thick skin, you may peel them partially or completely. For English cucumbers, leave the skin on.
Using a sharp knife or mandoline slicer, cut the cucumbers into very thin, translucent rounds.
If using, add the thinly sliced red onion to the bowl with the cucumbers.
In a separate bowl, whisk together the white vinegar, water, sugar, salt, black pepper, and optional Dijon mustard until sugar and salt are completely dissolved.
Pour the dressing over the cucumbers and onions, ensuring all slices are evenly coated.
Add the chopped fresh dill, optional parsley, and caraway seeds if using. Gently toss to distribute throughout the salad.
Cover and refrigerate for at least 30 minutes before serving, though 1-2 hours is preferable to allow flavors to fully develop.
Before serving, give the salad a final gentle toss to redistribute the dressing.
