Cut beef diagonally across the grain into 1/4-inch-wide strips; cut the strips into 2-inch pieces.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add beef to pan; sauté 2 minutes or until lightly browned. Remove beef from pan; place in a medium bowl, and keep warm. Add cremini mushrooms and onion to pan; sauté 4 minutes. Add cremini mushroom mixture to beef.
Melt butter in pan over medium heat. Add flour. Cook 1 minute, stirring with a whisk. Gradually add broth, stirring constantly. Cook 1 minute or until thickened and bubbly, stirring constantly.
Add beef mixture, sherry, salt, and pepper to pan; bring to a boil. Reduce heat, and simmer 4 minutes. Remove from heat; let stand 30 seconds. Stir in sour cream.
Combine noodles and minced parsley. Serve beef mixture over noodles. Garnish with parsley sprigs, if desired.
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