Preheat oven to 250 degrees F. Cube the loaf of bread and place on 1 or 2 baking sheets in single layers. Bake for 20-25 minutes until the bread is toasty, but not too hard. When bread is finished baking, increase oven temperature to 350.
Grease an 8x10 or 9x13 baking dish and set aside.
In a saucepan, combine the vegetable broth, vegan butter, celery, onion, and mushrooms. Melt the butter and bring to a simmer. Simmer for 10-12 minutes, until vegetables are tender.
Add the salt, sage, poultry seasoning, and pepper. Stir until combined and remove from head.
Place toasted bread in a large bowl. Pour butter mixture over the bread and mix well until all bread pieces are coated.
Transfer bread to prepared dish and bake in preheated oven for 40 minutes, or until bread no longer seems wet.