Wash cucumbers. Peel if using regular cucumbers.
Slice cucumbers very thin with a knife or mandoline.
Thinly slice red onion. Separate the rings.
Place cucumbers and onions in a large bowl.
In a small bowl, whisk sour cream, mayo, vinegar, sugar, dill, garlic powder, salt, and pepper until smooth.
Pour dressing over cucumbers and onions.
Toss gently to coat everything.
Cover and refrigerate for at least 1 hour.
Stir before serving. Drain excess liquid if needed.
Transfer to serving bowl and serve cold.