Preheat oven to 375°F (190ºC). Grease a 9x13-inch baking dish and set aside.
In a large skillet over medium heat, brown ground beef with the diced onion until meat is cooked through, about 8 minutes. Drain excess fat. Add corn, chili powder, cumin, garlic salt, and pepper to the ground beef mixture. Cook for 3 minutes. Remove from the heat.
In a large mixing bowl, combine cornbread mix, eggs, milk, sour cream, and melted butter. Mix until just combined.
Pour half the cornbread mixture into the prepared baking dish. Layer the beef mixture evenly over the batter. Do not dump the meat directly in the center as this will displace the cornbread batter.
Sprinkle 1 cup of cheese over top along with the drained diced tomatoes. Pour the remaining cornbread batter over the layers and spread evenly. Sprinkle the remaining cheese on top.
Bake for 35-40 minutes or until the cornbread is golden brown and a toothpick inserted into the bread comes out clean. Let stand for 10 minutes before serving.
Notes
Leftovers: Cornbread Casserole should be stored in an airtight container in the refrigerator for up to 3 days.To Reheat: Place a serving in the microwave and heat for 1-2 minutes, or until warmed through. For crispier leftovers, you can reheat in the oven at 350°F for about 10 minutes.To Freeze: Freeze the casserole by wrapping it tightly with plastic wrap and aluminum foil, or placing it in a freezer-safe container. It can be frozen for up to 3 months. To reheat, thaw overnight in the fridge and bake at 350°F for 20-25 minutes until heated through.