Heat oven to 350°F. Make mac & cheese as directed on box.
Meanwhile, in 10-inch skillet, heat beef and green onions over medium-high heat, stirring frequently, until beef is brown; drain. Stir in sloppy joe sauce. Cook 2 to 3 minutes, stirring occasionally, until thoroughly heated.
Spoon mixture into ungreased 1 1/2-quart casserole. Spoon mac & cheese evenly over meat mixture. Sprinkle with cheese.
Bake 15 to 20 minutes or until cheese is melted and mixture is hot.