In a large pot over medium heat, melt the butter. Add the finely chopped onion and cook for about 2 minutes until soft.
Add the sliced mushrooms and cook until they turn dark brown, approximately 10 minutes.
Stir in the garlic, dill, paprika, salt, pepper, and thyme. Cook for an additional 2 minutes.
Pour in the broth, soy sauce, and lemon juice. Bring to a boil, then simmer for 5 minutes.
In a small bowl, whisk the milk and flour together until smooth.
Gradually add the milk mixture to the soup, stirring constantly. Cook over medium heat for 20 minutes, until thickened.
Adjust salt and pepper to taste.
Serve hot, topped with a dollop of sour cream, chopped fresh parsley, and a side of baguette bread.