Combine yogurt, vinegar, garlic, dill, salt and pepper in a large bowl and stir until well combined.
Add the cucumbers and pickled red onions to the bowl. Toss to coat in the yogurt mixture.
Cover and chill for 20 minutes before serving. (**see note)
Notes
*For quick pickled red onions:Very thinly slice a small red onion and place in a small bowl (use a mandoline if you have one). Combine 1/2 cup apple cider vinegar, 1 cup warm water, 1 tablespoon sugar and 1 teaspoon kosher salt in another bowl or cup, stir to dissolve. Pour the mixture over the onions and let sit for 30 minutes to 1 hour before using.**Stir gently before serving if salad looks a little watery or separated. It's just the cucumbers giving off some water as they sit. You can avoid this by waiting to season with salt and pepper until right before serving.