Place sliced cucumbers in a colander and sprinkle with salt; let sit for 15 minutes to drain excess moisture.
Pat cucumbers dry with paper towels to ensure a creamy dressing consistency.
In a large mixing bowl, whisk together sour cream, white vinegar, sugar, chopped dill, and black pepper until smooth.
Add the drained cucumbers and sliced red onions to the bowl with the dressing.
Toss gently until all vegetables are evenly coated.
Cover and refrigerate for at least 30 minutes before serving to allow flavors to meld.