Melt butter in a large pot over medium heat. When the butter is melted, add onions and mushrooms and cook, stirring occasionally, until onions are soft and translucent, about 8 minutes. Add the stock, white wine, dill, thyme, paprika, Worcestershire and salt. Bring to a boil over high heat. Reduce to a simmer over low heat and cook, stirring occasionally, until the liquid reduces by ⅓, about 10 minutes.
In a small bowl, whisk flour into the milk until smooth. Add the milk mixture to the soup and cook, stirring occasionally, until the soup begins to thicken, about 10 more minutes.
Over low heat, slowly stir in the sour cream, and lemon juice until fully incorporated.
Divide the soup into four bowls and top with fresh parsley to serve.
