Please do not use anything but fresh mushrooms and butter in this.
Saute the mushrooms and chopped onion in 4 tablespoons of butter. Cook slowly for 20 minutes, stirring regularly. Add chicken stock.
Melt over low heat the other 4 tablespoons butter. Add the flour and cook for 5 minutes, stirring. Add milk or cream slowly and stir until thickened and smooth. Add salt and pepper. Stir in the mushrooms, onion and chicken stock. Slowly add the sour cream, which has been brought to room temperature.
Serve garnished with fresh, chopped parsley.