Place the whole, unpeeled potatoes in a large pot and cover with cold, salted water by 1 inch. Bring to a boil over high heat, then reduce to a simmer and cook for 20-25 minutes, until a knife slides through the center with no resistance. Drain and let sit until cool enough to handle but still warm.

While the potatoes cook, place the bacon in a cold large skillet. Turn the heat to medium and cook, stirring occasionally, until crispy and the fat is rendered, about 8-10 minutes. Use a slotted spoon to transfer the bacon to a paper towel-lined plate. Pour off all but 1/4 cup of the rendered fat from the skillet.

Add the diced onion to the skillet with the reserved bacon fat and sauté over medium heat until softened and translucent, about 3-4 minutes.

Add the apple cider vinegar, water, sugar, and Dijon mustard to the skillet. Whisk together, scraping up any browned bits from the bottom. Bring to a gentle simmer and cook for 1-2 minutes until the sugar dissolves. Season with a generous pinch of salt and a few cracks of black pepper.

Once the potatoes are cool enough to handle, peel off the skins using a paring knife (they should slip off easily). Slice the warm potatoes into 1/4-inch thick rounds and place them in a large mixing bowl.

Pour the hot dressing over the warm potato slices. Add the crispy bacon and half of the fresh parsley. Gently toss with a rubber spatula until all potatoes are well coated.

Let the salad rest at room temperature for at least 15-20 minutes before serving to allow the potatoes to absorb the dressing. Taste and adjust seasoning with more salt, pepper, or a splash of vinegar if needed. Garnish with the remaining parsley and serve warm.