Boil the water, put peeled and cut potato into quarters and cook until ready.
Finely chop the onion. In a skillet, melt the margarine and add the onions. Sauté until golden, add sliced mushrooms and a couple of tablespoons of water from the potatoes, stir-fry.
When the potatoes are cooked, add onions and mushrooms, season with salt and cook for 5 minutes. Separate the vegetable broth and the vegetables. Place the broth in a separate pot. Blend the vegetables separately and add back into the broth. Carefully add sour cream, put on the heat and bring to a boil.
Pour into hot soup bowls, garnish with parsley and freshly ground black pepper.