1. Adjust an oven rack to the middle position and heat the oven to 450 degrees.
2. For the tamale filling: Heat the oil in a 12-inch skillet over medium heat until shimmering. Add the onion, chili powder, and 1/2 teaspoon salt and cook until the onion is softened, about 5 minutes. Stir in the garlic and cook until fragrant, about 30 seconds.
3. Stir in the ground sirloin, beans, and tomatoes, and bring to a simmer, breaking up the meat with a wooden spoon, about 5 minutes. Stir the cheddar and cilantro into the filling and season with salt and pepper to taste.
4. For the cornbread topping: Meanwhile, whisk the flour, cornmeal, sugar, baking powder, baking soda, and salt together in a large bowl. In a separate bowl, whisk the buttermilk and egg together. Stir the buttermilk mixture into the flour mixture until
5. Dollop the cornbread batter evenly over the filling and spread into an even layer. Bake until the cornbread is cooked through in the center, 10 to 15 minutes. Serve.