Preheat the oven to 400°F. Heat the olive oil in a large oven-safe skillet over medium heat.
Add the diced onion and cook for 3 minutes until it softens. Add the ground turkey and break it up with a spoon. Cook until no longer pink.
Stir in the taco seasoning, black beans, and tomato sauce. Let the mixture simmer for 5 minutes so the flavors combine.
In a mixing bowl, whisk together the cornmeal, flour, baking powder, and salt. Add the milk, egg, and vegetable oil. Stir until the batter is just combined and smooth.
Pour the cornbread batter evenly over the turkey mixture in the skillet. Use a spatula to spread it gently if needed.
Transfer the skillet to the oven and bake for 20 minutes, or until the cornbread topping is golden and a toothpick inserted in the center comes out clean. Let it rest for a few minutes before serving.