Heat the butter over low to medium heat in a large saucepan and sauté the onions until translucent, do not brown.
Add the parsley cook for 1 more minute.
Add the mushrooms and increase the heat to medium. Cook until they start to release liquid, stirring often.
Season with 1 teaspoon salt and a bit of pepper.
Add the broth, bring to a boil and cover.
Simmer for 20 minutes.
In a small bowl, whisk the sour cream with the flour until well blended.
Gradually add it to the cooked soup, whisking vigorously after each addition until it is fully dissolved before adding more. This is to prevent lumps.
Bring the soup to a quick boil and continue whisking. It will be quite thick.
Add more broth if you prefer a thinner soup.
Season with a dash of white vinegar, salt and pepper.
Serve hot with a boiled potato in each bowl.