Preheat the oven to 350°F (180°C), or 160°C if using a fan oven.
In a 12-inch (30 cm) oven-proof skillet, over medium-high heat, heat olive oil then add the ground beef and onion, breaking it apart, and cook until browned (about 10 minutes). Add garlic, and cook for 30 seconds.
Add taco seasoning, and 2 tablespoons of water and cook for 2 minutes.
Add diced tomatoes with green chilies, pinto beans, and corn to the meat mixture. Stir and cook together for about 3 minutes then remove from heat. Taste and add salt if needed, to taste.
If you are not using an oven-proof skillet, transfer the mixture to a 9x13 inch baking pan (approx 33 x 22 cm). Then sprinkle the cheese all over the meat mixture.
To prepare the cornbread mixture, in a bowl, combine the corn muffin mix with the egg, milk, and melted butter.
Spoon the cornbread mixture all over the meat mixture as evenly as possible.
Bake in the preheated oven for 25-30 minutes or until puffed and slightly browned at the top. Remove from the oven, allow to cool for 5 minutes then serve with chopped cilantro and sour cream if desired.