Heat oven to 350°F. Lightly spray 13x9-inch glass baking dish with cooking spray.
In 10-inch skillet, heat uncooked pasta and sauce mix (from Hamburger Helper box), uncooked elbow macaroni, milk and hot water to boiling over medium-high heat. Reduce heat; cover and simmer 10 to 12 minutes, stirring occasionally, until pasta is tender. Remove from heat. Stir in mushrooms and chili. Spread mixture in baking dish. Sprinkle with cheese.
Bake 10 to 14 minutes or until mixture is hot and bubbly and cheese is melted. If desired, broil 3 to 5 minutes to brown cheese. Top with crushed red pepper flakes. Cool 5 minutes before serving.