In a large skillet, cook beef and onion together until beef is cooked through and onion is soft and translucent. Sprinkle flour over the mixture and cook for 1 minute, stirring constantly. Stir in tomatoes and bring to a boil; remove from heat.
Pour mixture into an ungreased 1 ½-quart casserole dish; spread out into an even layer and sprinkle with cheese. Arrange French fries in a single layer on top.
Bake for about 25 minutes or until French fries are golden brown.