Partially freeze steak and slice diagonally across grain into 3" strips. (When we first made this recipe we only had hamburger in the house. We later made it with steak and found we preferred it with hamburger.)
Cook mushrooms, onion and garlic over medium-high heat until browned, stirring frequently. Add flour and next four ingredients, stirring well. Bring to a boil; reduce simmer 15 minutes, stirring occasionally.
Stir in sour cream; cook until heated throughly (do not boil.) Serve over warm angelhair pasta and garnish with parsley.