Thinly slice the cucumbers into ⅛-inch rounds using a sharp knife or mandoline.
Thinly slice the onion into half-moons.
In a large bowl, combine cucumber and onion slices. Sprinkle with kosher salt and toss gently to coat. Let sit for 10 minutes to draw out excess moisture.
Meanwhile, in a small bowl, whisk together white vinegar, granulated sugar, and black pepper until the sugar dissolves completely.
Drain any liquid that has accumulated from the cucumbers and onions.
Pour the vinegar dressing over the cucumber mixture and toss until evenly coated.
Stir in chopped fresh dill if using.
Cover and refrigerate for at least 1 hour before serving to allow flavors to develop.
Toss gently once more before serving chilled.
