Cut beef across grain into strips about 1 1/2x1/4 inch.
In 12-inch skillet, melt butter over medium heat. Add mushrooms; cook, stirring occasionally, until mushrooms are tender; remove from skillet.
In same skillet, cook beef until brown. Stir in milk, sherry, cooked mushrooms, uncooked pasta and sauce mix (from Hamburger Helper box). Heat to boiling, stirring occasionally. Reduce heat; cover and simmer about 10 minutes, stirring occasionally, until pasta is tender.
Remove from heat; stir in sour cream. Let stand 5 minutes (sauce will thicken as it stands). Sprinkle with parsley just before serving.