Slice the cucumbers into thin rounds, about ⅛-inch thick. A mandoline works best for even slices.

Place the cucumber slices in a colander set over a bowl. Sprinkle with the salt and toss to coat. Let sit for 30 minutes at room temperature.

After 30 minutes, you'll see liquid released from the cucumbers. Rinse the slices briefly with cold water to remove excess salt, then pat them very dry with paper towels or a clean kitchen towel.

In a medium bowl, whisk together the sour cream, vinegar, sugar, and black pepper until smooth.

Add the dried cucumber slices, sliced onion, and chopped dill to the dressing. Stir gently until everything is well coated.

Cover the bowl and refrigerate for at least 1 hour (or up to 4 hours) to allow flavors to meld. Stir once more before serving.