Preheat oven to 400°F. In a 10-inch oven-proof skillet, cook beef and onion over medium-high heat until no longer pink, about 5-7 minutes. Drain off excess grease. Stir in enchilada sauce, diced tomatoes and green chilies, chili powder, and salt. Remove from heat.
Sprinkle with half of the cheese.
In a separate bowl, stir together the corn muffin mix, milk, egg, and remaining cheese. Pour over the beef filling in the skillet. Use a spatula to spread the topping in an even layer, covering and sealing in the meat mixture so that it doesn't bubble through the top.
Bake for 22-24 minutes, or until cornbread is golden brown and a toothpick inserted in the top of the crust comes out clean.
Notes
If you don't have a cast ironskillet (or other oven-proof skillet), or if you plan to prepare the pie in advance for freezing, you can assemble the pie in an 8-inch or 9-inch square baking dish, or in a deep-dish pie plate.
This recipe isnot gluten-free since the cornbread mix is not gluten-free. If you can find a gluten-free mix and a gluten-free enchilada sauce, the rest of the ingredients are naturally gluten-free.
Garnish individual servings of this easy tamale pie with chopped fresh cilantro, lime wedges, dollops of sour cream, or a spoonful of pico de gallo for even more bright flavor.