Preheat your oven to 400 degrees F (200 degrees C) and grease a 9×13 baking pan with non-stick spray.
In a 12-inch skillet, melt the unsalted butter over medium-high heat. Once melted, add in the diced celery and onion. Stir them together, allowing them to cook for about 10 minutes.
Remove the skillet from the heat and pour the sautéed celery and onion mixture into a large mixing bowl, including any remaining melted butter from the skillet.
In the same mixing bowl, add in the small diced toasted white bread, crumbled cornbread, fresh sage, poultry seasoning, kosher salt, and black pepper. Gently stir these ingredients together until well combined.
In a separate small mixing bowl, whisk together the eggs and milk. Pour this mixture over the bread mixture in the large bowl and stir to combine.
Drizzle in the turkey or chicken stock, adding just enough to wet the bread mixture without making it soggy.
Transfer the stuffing mixture into the prepared baking dish, spreading it evenly.
Place the dish in your preheated oven and bake for around 40 to 50 minutes until the top is lightly browned.
Remove the stuffing from the oven and let it sit for a few minutes before serving.
