Partially freeze steak; trim fat from steak. Slice steak diagonally across the grain into 1/4 inch wide strips; cut strips into 2 inch pieces.
Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add steak strips, sliced mushrooms, and chopped onion; saute 6 minutes or until steak is done and mushrooms are tender. Remove steak mixture from skillet. Drain well and set aside. Wipe drippings from skillet with a paper towel.
Combine bouillon granules and hot water, stirring well. Melt butter in skillet over medium heat, and add flour. Cook, stirring constantly with a wire whisk, 1 minute. Gradually add bouillon mixture, stirring constantly. Cook, stirring constantly, until thickened and bubbly.
Add steak mixture to flour mixture in skillet; add sherry, salt, and pepper. Bring to a boil; reduce heat, and simmer, uncovered, 6 minutes. Remove from heat, and let stand 30 seconds. Stir sour cream into steak mixture.
Combine cooked noodles and minced parsley; toss mixture well. Serve steak mixture over noodles. Garnish with parsley sprigs, if desired.