Heat oven to 350°F. Cook and drain noodles as directed on package.
While noodles are cooking, cook beef, onion, salt and pepper in 10-inch skillet, over medium heat 8 to 10 minutes, stirring occasionally, until beef is brown; drain.
In ungreased 3-quart casserole, mix noodles, beef mixture, sauerkraut, soup and 1 cup of the cheese.
Bake uncovered 45 minutes. Sprinkle with remaining 1 cup cheese. Bake uncovered 5 to 10 minutes or until cheese is melted. Sprinkle with red onion.