Bavarian potato salad

I picked this up when I was in Munich some 10 years ago, however, I've changed the recipe to be something like a potato soup.

5 - 6 medium sized potatoes
1 medium sized red or regular onion
1 clove of garlic
5 - 6 fresh mushrooms
1 dl vegetable stock (e. g. Oxo or Maggi)
pepper
light and dark soy sauce
balsamic vinegar

1. Wash, peel and dice the potatoes, then steam them.
2. In the mean time, peel and dice the onion coarsely.
3. Peel and slice the mushrooms thinly. Place along with the onion in a deep bowl.
4. Add the soy sauces and balsamic vinegar and let them stand for a few minutes in the marinade.
5. When the potatoes are ready, transfer them to the bowl as well, pour over the vegetable stock and stir well.
6. Serve warm.

The original recipe didn't have mushrooms in it and no marinade was made. Also I think 3 tbsp vegetable stock was used for the potato salad.

I think at some points, I'd put 2-3 dl stock and I'd get a soup that's a bit on the sour vinegary side, which I quite like.