Set your oven to 300℉ and gather all of your ingredients. Line a 13"x 9" baking dish with cooking spray.
Line 2 sheet pans with cubed bread, and pop into the oven for 10-15 minutes, just to lightly toast and dry out the bread. Note: Bread cubes do not need to be firm like croutons. Heat a small saucepan to medium high and then add a little of the melted butter. To the pan, sauté the onions just until translucent, about 5 minutes. Season with a little salt and pepper. Add in the celery, season with a little more salt and pepper, and cook an additional few minutes, just until the celery has softened.
Move the cooked vegetables to a large mixing bowl and allow to cool slightly.
Once the bread has been toasted, remove from the pans from the oven and increase the temperature up to 350℉.
To the large mixing bowl, add the toasted bread cubes, drizzle with the remaining melted butter, all of the herbs and the remaining salt and pepper, and give everything a toss to coat.
If you haven't done so already, vigorously whisk the eggs using a fork, then pour the eggs and the chicken stock over the bread cubes. Toss everything together and pour the mixture into a prepared baking dish.Bake at 350℉ for 1 hour, until the top is a light golden brown and the juices beneath the surface are bubbling (this is why I prefer using a clear glass baking dish over a ceramic one, in this case).Serve warm.