Prepare mashed potatoes according to package directions. Meanwhile, in a large skillet, cook beef and onion over medium heat until meat is no longer pink; drain. Add the tomatoes, corn, olives, taco seasoning, chili powder, salt and garlic powder. Bring to a boil; cook and stir for 1-2 minutes.
Transfer to a greased 2-1/2-qt. baking dish. Top with 3/4 cup cheese. Spread mashed potatoes over the top; sprinkle with remaining cheese. Bake, uncovered, at 350° for 12-15 minutes or until cheese is melted.
You may substitute 4-1/2 cups hot homemade mashed potatoes for the instant potatoes if desired.