In 10-inch skillet, cook beef over medium-high heat 5 to 7 minutes, stirring frequently, until thoroughly cooked; drain.
Stir in hot water, milk, sauce mix and uncooked pasta (from Hamburger Helper box), onion, frozen green beans and mushrooms. Heat to boiling, stirring occasionally.
Reduce heat; cover and simmer 10 minutes, stirring occasionally. Remove from heat; uncover and let stand 5 minutes (sauce will thicken as it stands). Stir before serving.