Step 1: Place the potatoes in a large pot and cover with salted water. Bring to a boil and cook for 15-20 minutes until tender. Drain and let cool slightly.
Step 2: While potatoes cook, fry the diced bacon in a large skillet over medium heat until crispy. Use a slotted spoon to remove bacon to a paper towel, keeping the fat in the pan.
Step 3: Add the red onion to the bacon fat and sauté for 3-5 minutes until translucent and softened.
Step 4: Whisk in the apple cider vinegar, water (or broth), mustard, sugar, salt, and pepper. Bring to a simmer for 1-2 minutes.
Step 5: Peel (optional) and slice the warm potatoes into 1/4-inch rounds and place them in a large mixing bowl.
Step 6: Pour the hot dressing over the potatoes. Add half of the bacon and half of the parsley. Toss gently to combine without breaking the potatoes.
Step 7: Let the salad sit for 15 minutes to allow the potatoes to absorb the dressing. Garnish with remaining bacon and parsley before serving warm.