Preheat oven to 400 degrees F.
In a large skillet over medium-high heat, brown ground beef and onion until no longer pink. Drain.
Return meat to the skillet and stir in salt, pepper, diced tomatoes, ¾ cup of cheese, and ketchup.
Spoon beef mixture into a 9-inch deep-dish pie plate that has been sprayed with cooking spray. Sprinkle remaining ¼-cup cheese over top of beef mixture.
In a medium bowl, combine corn muffin mix, egg, and milk and mix well. Spoon batter over top of pie.
Bake for 25-30 minutes, or until top is golden brown. Let stand about 5 minutes before serving.