Mix the Greek yogurt, white wine vinegar, garlic, dill and salt and pepper to make the dressing. For best results, let the dressing sit in the fridge for an hour for the garlic to mellow.
Add the sliced cucumber and onions to a large bowl. Add the dressing then toss and serve immediately.
* for best results eat this salad immediately. Cucumber is very watery, and the dressing will get very thin if left too long before serving. To make ahead of time leave the dressing off and dress the salad just before serving.