Bring a large pot of salted water to a boil.

Add the chopped potatoes and cook for about 15 to 20 minutes, or until they're tender but still holding their shape.

In another pan, sauté the diced onion in the reserved bacon grease for around 3 to 5 minutes, or until the onion becomes translucent.

Stir in the flour and cook for 1 minute to help thicken the dressing.

Add the vinegar, water, ground mustard, and sugar to the pan, seasoning with salt and pepper. Reduce the heat and let it simmer for about 2 minutes.

In a large bowl, combine the drained potatoes, chopped celery, and the warm dressing.

Gently toss everything together, then finish with crumbled bacon and a sprinkle of parsley on top.