Heat oven to 350°F. Line cookie sheet with foil.
In 10-inch skillet, cook beef over medium-high heat, stirring frequently, until brown; drain. Stir in hot water, milk, sauce mix and uncooked pasta (from Hamburger Helper box). Heat to boiling, stirring occasionally.
Reduce heat. Cover; simmer about 10 minutes, stirring occasionally, until pasta is tender. Remove from heat; uncover and allow sauce to thicken, about 5 minutes.
Meanwhile, clean mushrooms by gently wiping outsides of caps with damp paper towel or soft brush. Pop out stems. With teaspoon, scrape gills until undersides of caps are mostly clean. Place mushrooms on cookie sheet. Spoon stroganoff mixture into each mushroom. Top with cheese.
Bake 10 to 15 minutes or until mushrooms are cooked and cheese is melted. Top with parsley.