In soup pot over medium heat, heat oil. Add shallots and cook, stirring occasionally, until softened, 2 minutes. Add button mushrooms and cremini mushrooms, increase heat to medium-high and cook, stirring occasionally, until mushrooms release liquid and begin to brown, about 8 minutes. Stir in garlic and cook 30 seconds. Add broth, Worcestershire sauce, mustard, 3/4 teaspoon salt and pepper; bring to boil. Add egg noodles and boil gently, uncovered, until noodles are nearly tender, 5 minutes.

In pitcher or medium bowl, whisk milk, 1/3 cup sour cream and flour until flour dissolves. Ladle 1/2 cup broth from pot into milk mixture and whisk well then pour milk mixture into pot. While stirring, bring to gentle boil then lower heat and simmer until thickened, 2 minutes. Season with additional salt, to taste.

Serve garnished with dollop of remaining sour cream and parsley.