Prepare the Cucumbers:
Thinly slice the cucumbers using a knife or mandoline slicer for uniform pieces.
Sprinkle the cucumber slices with a pinch of salt and let them sit in a colander for 15–20 minutes to draw out excess water. Pat dry with a paper towel.
Prepare the Dressing:
In a medium bowl, whisk together sour cream, mayonnaise (if using), white vinegar, sugar, dill, salt, and pepper until smooth and creamy.
Assemble the Salad:
In a large bowl, combine the cucumbers and red onion slices. Pour the dressing over the vegetables and toss until well coated.
Chill and Serve:
Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld. Serve cold.