Scrub the potatoes well. Place them in a large pot, cover with cold salted water, and bring to a boil. Cook for 20-30 minutes, or until easily pierced with a fork. Do not overcook.

While the potatoes cook, prepare the vinaigrette. In a medium skillet, cook the bacon lardons over medium heat until crispy. Remove the bacon pieces with a slotted spoon and set them aside on a paper towel-lined plate. Leave the rendered bacon fat in the skillet.

Reduce the heat under the skillet to low. Add the apple cider vinegar, water/broth, sugar, Dijon mustard, salt, and pepper to the bacon fat. Whisk well and bring the mixture to a bare simmer, stirring constantly until the sugar is dissolved, about 2 minutes. Remove from heat.

Drain the potatoes immediately. While they are still hot, slice them into 1/4-inch thick rounds directly into a large bowl. Immediately pour the warm bacon vinaigrette over the warm potatoes. Gently toss to coat everything evenly.

Gently fold in the sliced red onion and the chopped fresh parsley. Taste and adjust seasoning if necessary (you may need a pinch more salt or vinegar).

Let the salad sit at room temperature for at least 15 minutes to allow the potatoes to absorb the dressing. Just before serving, sprinkle the reserved crispy bacon pieces over the top.