Wash the potatoes, put them in a saucepan, cover with water. Add caraway seeds and salt and cook for 25 minutes. Drain, let cool a little, peel and cut into slices.
Peel the onions, cut in half, cut into fine strips and add to the potatoes.
Mix the vinegar with the broth and mustard, season with salt and pepper. Finely puree 1 sliced potato with the oil in a tall container. Possibly add a little more oil. Add to the potato salad, mix well and let it steep.
If you like, sprinkle a few herbs over it at the end.