Place cucumbers in a large bowl. Sprinkle with salt and vinegar. Cover and let stand for 30 minutes. Pour off liquid.
In a separate bowl, whisk together sour cream, mayonnaise, sugar and pepper until smooth.
Pour dressing over the drained cucumbers. Stir in parsley or dill and sliced green onions.
Cover and chill until ready to serve.
Notes
Use a mandoline slicer to quickly cut the cucumbers into equal rounds. Make sure that you use the wider setting on the mandoline so that your cucumbers are not paper-thin. The thicker rounds stand up to the dressing and stay crisp for longer.
For the best flavor and texture, use full-fat mayo and sour cream rather than the lower-fat alternatives.
If time allows, let the cucumbers sit and marinate with the other ingredients for about 15-30 minutes before serving. Do not prepare the salad too far in advance, though. It’s best when enjoyed within about 4 hours so that the cucumbers don’t get soggy.