Preheat your oven to 400°F (200°C).
In a large skillet over medium heat, cook ground beef and diced onion until browned and cooked through. Drain excess grease.
Stir in chili powder, cumin, salt, and black pepper. Mix well.
Add corn, black beans, and diced tomatoes. Stir to combine and simmer for 5 minutes.
Transfer the beef mixture to a greased 9-inch square baking dish.
In a separate bowl, prepare the corn muffin batter by mixing the corn muffin mix with the egg and milk until just combined.
Spread the corn muffin batter evenly over the beef mixture.
Sprinkle shredded cheddar cheese on top.
Bake for 20 to 25 minutes, or until the top is golden and a toothpick inserted into the cornbread comes out clean.
Let cool slightly before serving.