Fit a large saucepan or pot with a steamer basket, and fill with water up to the bottom of the steamer. Bring to a boil, add potatoes and cook until just tender, 12 to 15 minutes. Make sure that the water doesn’t boil away. Spread the potatoes out over a baking sheet to cool to room temperature.
Whisk Green Mountain Creamery Greek yogurt, mayonnaise, vinegar, sugar (if using), salt and pepper in a large bowl. Add the potatoes, eggs, celery and onion and toss together until the potatoes are completely coated in the dressing.
Transfer to a serving bowl and garnish decoratively with red bell pepper, olives and parsley if using. Serve immediately or refrigerate up to 8 hours.