Slice the cucumbers and red onion thinly using a sharp knife or mandoline.
Place the sliced cucumbers and onion in a colander set over a bowl. Sprinkle with salt and toss to coat. Let sit for 15 minutes to draw out excess moisture.
Pat the cucumber and onion mixture dry with paper towels or a clean kitchen towel.
In a medium bowl, whisk together the mayonnaise, white vinegar, sugar, and black pepper until smooth.
Add the dried cucumber and onion to the dressing along with the chopped fresh dill. Gently toss until everything is evenly coated.
Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld.
Give the salad a quick stir before serving. Adjust seasoning with additional salt or pepper if needed.
