Heat the oven to 350°F (175°C) and grease a 9 x 13-inch baking dish.

Brown the ground beef and chopped onion in a skillet over medium heat, breaking the beef into small pieces. Cook for about 7–10 minutes, until the beef is no longer pink and the onion is soft.

Drain any excess fat from the skillet. Stir in the taco seasoning and the optional drained diced tomatoes with green chiles, then cook for 1–2 minutes.

In a large bowl, combine both corn muffin mixes, eggs, sour cream, whole-kernel corn, cream-style corn, green chiles, and 1 cup of shredded cheese. Stir only until combined, adding milk if the batter seems unusually thick.

Spread about half of the cornbread batter in the prepared baking dish. Use a spatula to create an even layer without pressing it down firmly.

Spoon the seasoned beef mixture evenly over the first cornbread layer. Keep the filling slightly away from the very edge when possible so the batter can surround it.

Spread the remaining cornbread batter over the beef layer. Sprinkle the remaining 1/2 cup of shredded cheese evenly across the top.

Bake for 40–45 minutes, until the top is golden and the center is set. A toothpick inserted into the cornbread portion should come out with only a few moist crumbs.

Let the casserole rest for 10 minutes before cutting. Finish with sliced jalapeños or chopped cilantro if desired, then serve warm.