Slice cucumbers thinly using a mandoline or sharp knife. If using regular cucumbers, peel and seed them first.
Place cucumber slices in a colander, sprinkle with a little salt, and let sit for 10 minutes to draw out excess water. Pat dry with paper towels.
In a medium bowl, whisk together Greek yogurt, sour cream, lemon juice, minced garlic, chopped dill, salt, pepper, and sugar until smooth.
Add the cucumber slices (and red onion if using) to the dressing. Toss gently until all slices are evenly coated.
Cover the bowl and refrigerate for at least 30 minutes to allow flavors to meld. Stir before serving.