Freshly chopped parsley, for garnish
1/4 cup sour cream
1/2 cup dry white wine
1/2 cup beef broth
2 tbsp. all-purpose flour
2 cloves garlic, minced
8 oz. sliced baby bella mushrooms
1 large onion, diced
kosher salt
3/4 pound London broil
3 tbsp. butter, divided
12 oz. fettuccine