In a large skillet, cook beef and onions over medium heat until meat is no longer pink; drain. Stir in the corn, soup, garlic, mustard, salt, pepper and basil. Bring to a boil. Reduce heat; cover and simmer for 10-15 minutes. Stir in the Parmesan cheese.
Spoon into a greased 13x9-in. baking dish. Top with mashed potatoes and cheese slices. Bake, uncovered, at 350° for 20-30 minutes or until bubbly.