Brown ground beef in a skillet over medium-high heat; drain. Stir in seasoning mix, tomato paste and water; heat and stir until blended.
Spoon into a lightly greased 13"x9" baking pan; stir in macaroni, corn, one cup cheese, and mushrooms, if desired. Bake, uncovered, at 350° for 45 minutes. Sprinkle with remaining cheese; return to oven until cheese melts.