Slice the red onion thinly using a sharp knife or mandoline.
Wash the cucumbers well, slice them thinly, then gently pat them dry with paper towels to remove excess water.
In a medium bowl, whisk together sour cream, white vinegar, fresh dill, sugar, salt, and garlic powder until fully incorporated.
Fold in the sliced cucumbers and red onion carefully, making sure they are evenly coated with the dressing.
Sprinkle with extra fresh dill and freshly ground black pepper if you like.
Serve the salad right away or cover and chill it for better flavor. Keep leftovers refrigerated in a covered container for up to three days.