Preheat oven to 400°F. Grease an 8-by-8-inch baking pan with nonstick cooking spray and set aside.

In a large mixing bowl, combine flour, cornmeal, granulated sugar, baking powder, and salt.

In a separate bowl, whisk together eggs and buttermilk. Pour the wet ingredients into the dry ingredients and mix until combined.

Pour batter into the prepared pan and bake for approximately 20 minutes, or until a toothpick inserted into the center comes out clean. Allow the cornbread to cool completely before preparing the dressing.

Preheat oven to 350°F. Grease a 9-by-13-inch baking dish with nonstick cooking spray. Crumble the cooled cornbread directly into the dish.

Add chopped onion, celery, and vegetable oil to a skillet over medium heat. Cook for approximately 5 minutes, until the onion is tender. Spoon the cooked vegetables over the crumbled cornbread.

In a separate bowl, whisk together condensed cream of chicken soup, eggs, poultry seasoning, and ground black pepper. Use the condensed soup directly from the can without preparing it with water or milk.

Bake uncovered for 45–50 minutes, until the center is set and the top is lightly browned. For food safety, check that the center reaches 165°F. Allow to stand briefly before serving.