Boil the potatoes in salted water for about 25 minutes (depending on size, I usually use large ones, then the time is optimal).
In the meantime, cut the onions and bacon into small cubes. Leave the bacon in a pan until translucent. Then bring to the boil together with the onions in the broth. Remove from heat and set aside.
Peel the potatoes, cut into thick slices and place in a bowl.
Add the mustard, vinegar and oil to the bacon and onion broth and mix well. Season with salt and pepper. Put on the potatoes, mix and, if necessary, season again with salt and pepper.
I prefer vegetable broth, my mother took chicken broth.
3 tablespoons of vinegar is enough for me, my mother always took 4, just try it out.