Thinly slice the English cucumbers and red onion using a mandoline or sharp knife.
In a small mixing bowl, whisk together the sour cream, mayonnaise, white vinegar, sugar, salt, black pepper, and chopped dill until smooth.
Place the sliced cucumbers and onions in a large mixing bowl.
Pour the dressing over the vegetables and toss gently until every slice is evenly coated.
Cover the bowl and refrigerate for at least 1 hour to allow the flavors to develop and the cucumbers to chill.
Stir the salad gently one last time before serving chilled.