Spray a 2-quart baking dish with nonstick cooking spray or rub with softened butter.
Cut the sourdough loaf into 3/4" chunks. Toss the bread onto 1 or 2 large sheet pans in a single layer and toast in the oven for 15 minutes.
While the bread toasts, add the butter and olive oil to a large 12" skillet and melt over medium heat.
Add the onion and celery along with a pinch of salt and cook until the onions are translucent and tender for about 6 minutes.
Add in the garlic and fresh herbs and cook another minute, then remove from heat.
In a medium bowl, whisk together the chicken stock and egg.
Put the toasted bread in a large mixing bowl.
Add in the onion and herb mixture, chicken stock mixture, salt, and pepper.
Mix well until all the liquid is absorbed in the bread.
Transfer to a prepared baking dish, press the bread FIRMLY into the pan.
Sprinkle with additional herbs if desired.
Bake for 45-50 minutes, if it is browning too quickly, you can tent it loosely with foil.
This recipe works well with any crusty bread such as Sourdough, Italian, French, or Ciabatta. Day-old works the best!If you prefer to not toast the bread, cut it up and let it dry out overnight on a sheet pan.To prep this ahead: Prep as directed and cover with plastic wrap, refrigerate overnight. Take it out and leave it at room temperature for about an hour before cooking then move on with the baking.This recipe can be doubled by x2 all ingredients.