Place the potato chunks in a large pot and cover with cold water by about an inch. Add a generous pinch of salt and bring to a boil over high heat. Reduce heat to maintain a gentle boil and cook until potatoes are just tender when pierced with a fork, about 12-15 minutes. Drain well and transfer to a large mixing bowl.

While the potatoes cook, heat a large skillet over medium heat. Add the chopped bacon and cook, stirring occasionally, until crisp and browned, about 8-10 minutes. Use a slotted spoon to transfer the bacon to a paper towel-lined plate, leaving the rendered fat in the skillet.

Add the chopped onion to the bacon fat in the skillet. Cook over medium heat, stirring frequently, until the onion is softened and translucent, about 5-7 minutes. Don't let it brown too much—you want it tender but not crispy.

To the skillet with the onions, whisk in the apple cider vinegar, Dijon mustard, chicken broth, sugar, salt, pepper, and celery seed if using. Bring to a simmer and cook for 2-3 minutes, stirring occasionally, until the sugar dissolves and the mixture thickens slightly.

Pour the warm dressing over the drained potatoes in the mixing bowl. Add the cooked bacon and chopped parsley. Gently fold everything together until the potatoes are evenly coated with the dressing. Be careful not to mash the potatoes—you want them to maintain their shape.

Let the potato salad rest for 15-20 minutes at room temperature, folding gently once or twice during this time. This allows the potatoes to absorb more of the dressing and the flavors to meld together.

Taste and adjust seasoning if needed—you might want an extra pinch of salt or a splash more vinegar depending on your preference. Serve warm or at room temperature.