Preheat the oven 350°F.
In a large sauce pan, melt the butter over medium heat. Add the onion, celery, carrot, and mushrooms and cook until golden and soft, stirring often, about 10 minutes.
Place the toasted bread cubes in a large bowl. Add the vegetables, herbs, stock, egg, and salt and pepper. Place in a oven-proof baking dish and cover with foil. Bake for 30-40 minutes, remove foil and bake for an additional 10 minutes to crisp up the top. Enjoy!
