In a large saucepan over medium heat, melt butter. Add mushrooms and onion and sauté until tender, about 6 minutes. Add flour, salt and pepper and stir to combine.
Gradually stir in milk, broth and bouillon, bring to boil, and cook, stirring, 2 minutes. Reduce heat, stir in sour cream, and cook until heated through (do not boil). Garnish each serving of mushroom soup with parsley, if desired.