Heat oven to 375°F. Cut tops off peppers; discard tops. Remove seeds and membranes; discard. Place bell peppers on ungreased cookie sheet.
In 10-inch nonstick 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 6 to 8 minutes, stirring occasionally, until pasta is tender. Remove from heat. Spoon mixture into peppers. Top each with 1 tablespoon cheese.
Bake 15 to 20 minutes or until peppers are tender and cheese is melted. Sprinkle with red pepper flakes.