In a medium bowl, combine cucumber, radishes, shallot, dill, and chives.
In a separate bowl, whisk together buttermilk, mayonnaise, sugar, vinegar, salt, and pepper.
Pour the dressing over the cucumber mixture and toss to combine.
Cover and refrigerate for about 15-30 minutes or up to 4 hours.
Notes
Why is my cucumber salad watery? As the salad sits, the vegetables release liquid and thin the dressing. That’s why it’s best to enjoy the salad shortly after stirring it together. If you need to prep ahead, see my tip above.
Use a mandoline slicer to quickly cut the cucumbers and radishes into equal rounds. Make sure that you use the wider setting on the mandoline so that your vegetables are not paper-thin. The thicker rounds stand up to the dressing and stay crisp for longer.
Use whole buttermilk rather than low-fat buttermilk, since the whole buttermilk has a much thicker, richer texture and flavor. Be sure to shake the bottle well before using it, too!
If time allows, let the cucumbers and radishes 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 vegetables don’t get soggy.