Thinly slice the cucumbers and red onion using a sharp knife or mandoline.
Place the sliced vegetables in a colander set over a bowl or in the sink, sprinkle with salt, and toss to coat. Let drain for 15 minutes to remove excess moisture.
While the vegetables drain, whisk together the sour cream, white vinegar, sugar, and black pepper in a large mixing bowl until smooth.
Pat the drained cucumber and onion slices dry with paper towels or a clean kitchen towel to remove any remaining moisture.
Add the dried vegetables and chopped fresh dill to the sour cream mixture, gently folding until everything is evenly coated.
Cover and refrigerate for at least 30 minutes to allow flavors to meld. Stir gently before serving chilled.
