Preheat oven to 450 °F.
In a 12" or 14" oven proof skillet, sauté the onions in the oil until softened, about 3 minutes.
Add garlic and cook for another 30 seconds, then add in ground beef (or beans) and cook through.
Season mixture with chili powder, cumin and salt.
Add corn, tomatoes, and cheese, stir together well and flatten the mixture out evenly in the pan. Remove from heat.
Make the cornbread mix according to package directions and pour into the skillet, smoothing it evenly over the filling.
Bake for 15-17 minutes until cornbread is golden.