Heat oven to 375 degrees. Spray 2-qt. glass baking dish with nonstick cooking spray. In large skillet, cook ground beef and onion over medium-high heat until beef is thoroughly cooked, stirring frequently. Drain.
Reduce heat to medium; stir in soup. Cook until mixture comes to a boil, stirring occasionally.
Spoon beef mixture into sprayed baking dish. Spoon pizza sauce evenly over top. Arrange pepperoni over sauce. Sprinkle with cheese. Arrange potato nuggets over chees6e. Cover with foil.
Bake for 30 minutes. Remove foil; bake additional 15 to 20 minutes or until thoroughly heated.