Place the sliced cucumbers and red onion in a colander and sprinkle with salt. Let sit for 10 minutes to draw out excess moisture, then rinse and pat dry with paper towels.
In a medium bowl, whisk together sour cream, mayonnaise, vinegar, sugar, salt, and pepper until smooth.
Add the cucumbers and onions to the dressing and toss to coat evenly.
Stir in fresh dill if using. Cover and refrigerate for at least 30 minutes before serving to allow flavors to meld.
Serve chilled as a side dish.
Notes
For best results, use English cucumbers for fewer seeds and a crisper texture.