Heat oven to 450°F. In 12-inch nonstick skillet, cook beef and onion over medium-high heat about 8 minutes, stirring occasionally, until beef is thoroughly cooked; drain well. Sprinkle flour over beef mixture. Cook 1 minute, stirring constantly. Stir in tomatoes; heat to boiling. Remove from heat.
In ungreased 1 1/2-quart casserole, spread beef mixture. Sprinkle with cheese. Arrange frozen potatoes evenly in single layer on top.
Bake uncovered about 20 minutes or until potatoes are golden brown. Let stand 5 minutes before serving.