In large skillet, cook ground beef and onion over medium heat until beef is thoroughly cooked, stirring frequently. Drain.
Add pasta sauce and water; mix well. Bring to a boil. Stir in macaroni from dinner. Return to a boil. Cover; cook over medium heat for 6 to 8 minutes or until macaroni is tender and liquid is absorbed, stirring occasionally.
Stir in cheese powder from dinner; mix well. Stir in mushrooms. Sprinkle with Cheddar cheese; cover and cook 1 to 2 minutes or until cheese is melted.