Boil the potatoes as jacket potatoes for about 20 to 25 minutes. Peel warm, then let cool and cut into cubes or slices while cold.
For the dressing, peel the onion and cut it into cubes. Fry in the pan with the bacon - it can be a little crispy. Deglaze with the broth, then stir in the mustard and a dash or two of vinegar. Season to taste with salt and pepper. Let simmer a little. Then pour the dressing over the potatoes while still warm and stir. The potatoes can swim a bit. Let everything steep for a while, preferably one to several hours. The sauce will soak in over time.