Preheat oven to 400 degrees F. Coat an 8-inch-square baking dish with cooking spray.

Place tomatoes, ½ cup broth, chili powder, cumin, oregano, onion powder and salt in a blender. Blend until smooth. Transfer to a medium bowl; stir in chicken and black beans. Pour the mixture into the prepared baking dish.

Combine the remaining 2 cups broth and water in a large saucepan; bring to a boil. Pour in cornmeal (or grits) in a steady stream, whisking constantly. Bring to a simmer, whisking constantly. Reduce heat to medium-low and cook, whisking often, until thick, about 5 minutes. Remove from the heat.

Reserve 1 tablespoon scallions. Combine the remaining scallions, ½ cup Cheddar and egg in a small bowl. Quickly stir the cheese mixture into the hot cornmeal mixture until combined.

Spread the cornmeal mixture over the chicken mixture in the baking dish. Sprinkle with the remaining ¼ cup cheese. Bake until hot and bubbling, about 20 minutes. Sprinkle with the reserved 1 tablespoon scallions. Let stand for 10 minutes before serving.