Preheat the oven to 400 degrees F.
Heat a cast iron skillet over medium-high heat. Add the ground beef and chopped onions and cook, stirring frequently, until it is browned and crumbly, usually taking about 5 to 7 minutes. Once cooked, drain and discard any excess grease. Season the beef with salt and black pepper.
Stir in the taco seasoning and water into the cooked ground beef. Cook and stir the mixture over medium heat for about a minute, then promptly remove the skillet from the heat. Add the black beans, pinto beans, and crushed tomatoes to the skillet, combining all the ingredients well.
In a separate large bowl, beat the cream-style corn, cornbread mix, and egg until the mixture becomes smooth and well-combined.
Spread the cornbread mixture evenly over the ground beef mixture in the skillet, creating a delicious layer of cornbread goodness. Sprinkle the shredded Cheddar cheese on top, adding a luscious cheesy touch to the casserole.
Place the skillet in the preheated oven and bake for approximately 25 minutes or until the cheese is melted and the cornbread is set in the middle. The casserole should turn golden brown.
Serve with the toppings of your choice. Enjoy!