Heat the oven to 375°F (190°C). Coat a 9 × 13-inch baking dish with grease and set it aside.
Place the ground beef and diced yellow onion in a large skillet over medium heat. Cook for approximately 8 minutes, breaking up the meat as necessary, until the beef is completely browned and the onion has softened.
Drain away any excess grease from the skillet.
Add the frozen corn, chili powder, cumin, garlic salt, and black pepper to the cooked beef mixture. Stir thoroughly and continue cooking for another 3 minutes. Remove the skillet from the heat.
In a large mixing bowl, combine the corn muffin mix, eggs, whole milk, sour cream, and melted butter. Stir only until the ingredients are incorporated. Avoid excessive mixing so the baked cornbread remains light and tender.
Spread half of the prepared cornbread batter evenly across the bottom of the greased baking dish.
Distribute the cooked beef and corn mixture evenly over the batter rather than placing it in a single area. Scatter 1 cup of the shredded cheddar over the beef layer.
Arrange the drained diced tomatoes evenly over the cheese.
Carefully spread the remaining cornbread batter over the tomato layer. Sprinkle the final 1 cup of shredded cheddar evenly across the top.
Transfer the casserole to the preheated oven and bake for 35–40 minutes. It is ready when the cornbread is golden brown and a toothpick inserted into the bread portion comes out clean.
Allow the casserole to stand for 10 minutes after removing it from the oven. This resting period helps the layers set and makes the casserole easier to slice.