Melt the butter in a large pot over medium heat. Sauté the onions in butter for 5 minutes. Add the mushrooms and sauté for 5 more minutes (if using different mushrooms, ensure they are cooked - King Oysters take much longer than 5 minutes).
Stir in the dill, paprika, soy sauce and broth. Reduce heat to low, cover, and simmer for 15 minutes.
In a separate small bowl, whisk the milk and flour together. Pour this into the soup and stir well to blend. Cover and simmer for 15 more minutes, stirring occasionally.
Finally, stir in the salt, ground black pepper, lemon juice, parsley and sour cream. Mix together and allow to heat through over low heat, about 3 to 5 minutes. Do not boil. Serve immediately.