Preheat the oven to 250°F degrees and line two baking dishes with parchment paper.
Prep your onion, herbs, mushrooms, and celery.
Cut bread into 1 inch pieces or use your hands to tear them into pieces. Add the bread pieces to a parchment paper lined baking tray. Bake at 250°F for 50-60 minutes until bread is toasted (hard to the touch), but not burnt. Remove the bread from the oven and let cool.
Increase oven temperature to 350°F
While the bread cooks, add the oil to a medium skillet over medium heat and cook the onions until browned, stirring frequently. Then add in the celery and cook it for a few minutes until soft, then add in mushrooms and garlic and saute until soft, about 5 more minutes, stirring frequently. Once the vegetables are cooked. Turn the heat off and mix in the 1⁄4 cup melted butter and mix together well.
In a large bowl, add the bread, then add in the sauteed vegetables with butter, and mix together. Add in the parsley, sage, rosemary, thyme, salt and pepper and mix together again.
In a small bowl, whisk together vegetable broth and the egg. Add the broth mixture over the bread, all pieces of the bread and vegetables should be coated.
Coat your 8x11 (or larger) baking dish with butter. Then add in your bread mixture, spreading it out evenly (you want it to be loosely packed).
Cover the baking dish with tin foil and bake at 350°F for 30 minutes. Remove the foil and bake for another 20-30 minutes or until the crispy on top.
Let cool for 5-10 minutes, then ENJOY!