In a large colander set over a bowl, combine sliced cucumbers and red onion. Sprinkle with kosher salt and toss gently to coat. Let sit for 15 minutes to draw out excess moisture.

While vegetables rest, prepare the dressing: In a medium bowl, whisk together sour cream, white vinegar, sugar, chopped dill, and black pepper until smooth and well combined.

Rinse the salted cucumber and onion mixture under cold running water to remove excess salt. Drain thoroughly and pat dry with paper towels or a clean kitchen towel.

Transfer the drained vegetables to a large mixing bowl. Pour the sour cream dressing over them and gently fold everything together until evenly coated.

Cover the bowl with plastic wrap or transfer to an airtight container. Refrigerate for at least 30 minutes to allow flavors to meld.

Before serving, give the salad a gentle stir. Taste and adjust seasoning with additional salt or pepper if needed. Serve chilled.