Preheat oven to 375 degrees F (190 degrees C). Grease a 9x13 inch baking dish.
In a large skillet, brown the ground beef over medium-high heat. Drain off any excess grease.
Add the chopped onion and minced garlic to the skillet and cook until softened, about 5 minutes.
Stir in the diced tomatoes (undrained), rinsed and drained black beans, diced green chilies, chili powder, cumin, salt, and pepper. Bring to a simmer and cook for 5 minutes, stirring occasionally.
Pour the tamale filling into the prepared baking dish.
In a medium bowl, whisk together the cornbread mix, egg, milk, and vegetable oil until just combined. Do not overmix.
Pour the cornbread batter evenly over the tamale filling.
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.
Let stand for 5-10 minutes before serving.
Notes
Serve with your favorite toppings like sour cream, salsa, avocado, or shredded cheese.