Spray a 2 1/2 quart casserole dish or other ovenproof dish with cooking spray. Preheat your oven to 350 degrees.
Brown ground beef with onion in a large skillet over medium-high heat. Drain off excess fat.
Add the taco seasoning, chili powder, salt, and garlic. Saute for a minute. Stir in the beans, tomatoes, corn, and olives. Simmer for 2-3 minutes over medium heat.
Spoon the filling into the prepared dish. Spread mashed potatoes over the meat mixture. Top with the grated cheese.
Bake at 350° for about 25-30 minutes or until the cheese is melted and the filling is hot.