Begin by washing the cucumbers thoroughly. Cut them in half lengthwise, then slice them into half-moons about ¼-inch thick.
Place the sliced cucumbers in a large mixing bowl. Add the thinly sliced red onion and mix lightly.
In a separate bowl, whisk together the Greek yogurt, mayonnaise, chopped dill, lemon juice, garlic powder, salt, and pepper until smooth.
Pour the creamy dressing over the cucumbers and red onion. Gently toss until all the vegetables are well coated.
Taste and adjust seasoning with additional salt or pepper if desired.
Cover the bowl with plastic wrap and refrigerate for at least 30 minutes before serving to allow the flavors to meld together.
Serve chilled, garnished with additional dill if desired.