Wash and slice cucumbers into thin rounds (approximately 1/8-inch thick). If using standard cucumbers with thick skin, you may want to peel them first.
If using red onion, soak sliced onion in cold water for 10 minutes to reduce sharpness, then drain and pat dry.
In a large bowl, combine Greek yogurt, chopped dill, lemon juice, minced garlic, olive oil (if using), and honey (if using). Mix well until smooth and creamy.
Season the yogurt dressing with salt and freshly ground black pepper to taste.
Add cucumber slices, drained red onion (if using), chopped mint (if using), and chives (if using) to the bowl with the yogurt dressing.
Gently toss all ingredients until the cucumbers are evenly coated with the yogurt dressing.
Taste and adjust seasoning if necessary.
Transfer to a serving bowl and sprinkle with crumbled feta cheese and ground sumac if desired.
Chill the salad for at least 10 minutes before serving to allow the flavors to meld.
Serve cold as a refreshing side dish or light meal.
