Thinly slice the cucumbers into 1/8-inch rounds and slice the red onion into thin strands. Pat the cucumber slices dry with paper towels to remove excess moisture—this prevents the salad from becoming watery as it sits. Set both vegetables aside in a large bowl.

In a separate bowl, whisk together the sour cream, mayonnaise, white vinegar, sugar, salt, garlic powder, and black pepper until smooth and well combined. Fold in the fresh chopped dill gently to preserve its delicate texture. I like to taste the dressing at this point and adjust the seasoning—sometimes a tiny pinch more salt or vinegar makes all the difference depending on your cucumber variety.

Pour the creamy dressing from Step 2 over the prepared cucumbers and red onions in the large bowl. Gently toss everything together until the vegetables are evenly coated. Cover and refrigerate for at least 1 hour, which allows the flavors to meld and the cucumbers to release their juices slightly, creating a silky sauce. I find that tossing it gently halfway through chilling helps distribute the dressing more evenly.