Slice cucumbers thinly, about 1/8 inch thick. Place in a colander and toss with salt. Let sit for 10 minutes to draw out excess moisture.
In a medium bowl, whisk together sour cream, mayonnaise, vinegar, dill, sugar, and black pepper until smooth.
After 10 minutes, gently squeeze handfuls of cucumber slices to remove excess liquid. Do not rinse. Pat dry with paper towels.
Add cucumbers and red onion (if using) to the dressing. Stir gently until evenly coated.
Taste and adjust seasoning with more salt or vinegar if needed. Serve immediately or refrigerate for 30 minutes to allow flavors to meld.