Preheat the oven to 375°F (190°C). Grease a 9×9-inch baking dish with cooking spray or butter.
In a large skillet, heat 1 tablespoon olive oil over medium heat. Add the diced onion and minced garlic, and sauté for 2-3 minutes until softened.
Add the ground beef to the skillet. Cook, breaking it up with a spoon, until browned and cooked through, about 6-8 minutes. Drain excess fat if necessary.
Stir in the taco seasoning mix and 1/4 cup water. Cook for another 2 minutes until the seasoning is well incorporated and the mixture is slightly thickened. Remove from heat.
In a large mixing bowl, combine the Jiffy corn muffin mix, sour cream, milk, vegetable oil, and egg. Stir until just combined.
Fold in the thawed corn kernels, diced green chilies, and half of the shredded cheddar and Monterey Jack cheeses into the batter.
Spread the cooked ground beef mixture evenly in the prepared baking dish.
Pour the cornbread batter evenly over the beef layer and spread gently with a spatula to cover the beef.
Sprinkle the remaining shredded cheddar and Monterey Jack cheeses evenly on top.
Bake in the preheated oven for 30-35 minutes, or until the cornbread is golden brown and a toothpick inserted into the center comes out clean.
Remove from oven and let the casserole cool for 5 minutes before serving.
Garnish with fresh cilantro leaves if desired and serve warm.