Wash and thinly slice cucumbers into ⅛-inch rounds.
Place cucumber slices in a colander, sprinkle with ½ teaspoon salt, and let drain for 10 minutes.
While cucumbers drain, whisk together mayonnaise, sour cream, vinegar, sugar, and black pepper in a large bowl until smooth.
Pat cucumber slices dry with paper towels to remove excess moisture.
Add cucumbers, chopped dill, and red onion to the dressing mixture.
Gently toss until all ingredients are evenly coated with dressing.
Cover and refrigerate for at least 15 minutes before serving to allow flavors to meld.
Taste and adjust seasoning with additional salt or pepper if needed before serving.
