Prepare the Vegetables: Thinly slice the cucumbers and red onion. Place them in a large mixing bowl.
Make the Dressing: In a small bowl, combine the vinegar, sugar, salt, and ground black pepper. Mix well.
Dress the Salad: Pour the dressing over the sliced cucumbers and onion.
Toss to Combine: Toss the cucumbers and onion until well combined with the dressing. Serve the salad immediately or let it chill in the refrigerator for about 15 minutes to let the flavors meld together.
Notes
Cucumber: I used Persian cucumbers. You can also use English, Kirby, or garden cucumbers. Generally, there's no need to peel or remove the seeds as these cucumbers have thin skin and small seeds. However, if you find the skin too thick or waxed, feel free to peel—it's totally up to you.
Vinegar: For the vinaigrette, you can use various types of vinegar to suit your taste. I used cane vinegar, but white vinegar, apple cider vinegar, or rice vinegar also work well.