In a large skillet, cook beef and onion over medium heat until meat is no longer pink; drain. Transfer to a greased roasting pan. Stir in the noodles, corn, peas and mushrooms.
In a large saucepan, combine 2-1/2 cups cheese with the remaining ingredients. Cook and stir over low heat until cheese is melted. Pour over noodle mixture and mix well. Sprinkle with remaining cheese.
Bake, uncovered, at 350° for 30 minutes or until heated through. Yield: 12-14 servings.