Place the thinly sliced cucumbers and red onions in a large mixing bowl.
In a small separate bowl, whisk together the sour cream, apple cider vinegar, chopped dill, sugar, salt, and black pepper until the dressing is smooth and emulsified.
Pour the dressing over the cucumber and onion mixture.
Using a large spoon or spatula, toss the vegetables gently until every slice is thoroughly coated.
Serve immediately for maximum crispness, or cover and refrigerate for up to 30 minutes before serving to allow flavors to integrate.