Slice the red onion thinly using a sharp knife or mandoline. Rinse cucumbers well, slice them thinly, and then blot with paper towels to remove extra moisture.
Combine sour cream, white vinegar, minced dill, sugar, salt, and garlic powder in a medium bowl, mixing until smooth.
Add the prepared cucumbers and onions to the dressing and fold gently until all pieces are coated.
Top with extra fresh dill and sprinkle with black pepper if you like. Serve right away or cover and chill in the fridge before serving. Keep leftovers stored in an airtight container in the refrigerator for up to three days.
