Cook potatoes whole with 1 teaspoon salt in a large pot. This may take 30-40 minutes depending on the size of your potatoes. (If some potatoes are very large you may cut them in half to make them similar in size to the rest. Don't cut into more than 2 pieces.) Test with a fork after 20 minutes to avoid over-cooking. Drain when a fork slides easily to the center of the potato.
While potatoes are cooking fry the bacon strips in a fry pan until crisp over medium heat. Crumble when cool. Make the hard boiled eggs and cool in ice water. Prepare the celery, onion and cucumber. Be sure to remove all seeds and watery center of the cucumber before dicing. See Note 1.
Allow potatoes to cool 5-10 minutes. They will still be very warm. (You want to peel and cube them while they are warm so they will absorb the vinaigrette when poured over them.) See Note 2. Discard skins and cube potatoes into ½" dice in a large bowl.
When all potatoes are cubed pour ½ the vinaigrette over them and stir well. Pour the remaining vinaigrette over and stir again.
Allow potatoes to cool completely. (If making ahead refrigerate at this point and proceed with next steps when ready.).
When potatoes are cooled, add the celery, onion, cucumber, bacon and egg. Add the parsley as well if using.
Mix the Dressing ingredients and pour over the salad, stirring to coat completely.
Allow to sit at least 30 minutes to allow flavours to meld but 3-24 hours is even better!
Leftovers keep well in the fridge for 2-3 days.