Heat oven to 375°F. Spray 8-inch square (2-quart) 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 and bell pepper over sauce. Sprinkle with cheese. Arrange potato nuggets over cheese. Cover with foil.
Bake at 375°F. for 30 minutes. Remove foil; bake an additional 15 to 20 minutes or until thoroughly heated.