Oven Prep: Preheat the oven to 375F and set a large baking sheet aside. Grease a 9x13” baking dish and set aside as well.
Toast the Bread: Cut the bread into 1” cubes and spread evenly across the baking sheet. Bake in the oven for 15 to 20 minutes, flipping halfway, until the bread is toasted and dry. Set aside to let cool.
Vegetable Prep: Chop roughly half of the mushrooms into quarters or medium pieces and set aside. Place the rest of the mushrooms in a food processor with an S-blade attachment and pulse until they are finely chopped. Transfer the chopped mushrooms to a bowl, then add the onion and celery to the food processor. Pulse again until they are finely chopped; set aside.
Aromatics: Melt the butter in a large sauté pan or pot over medium high heat. Add all of the mushrooms to the pot and sauté for 5 minutes, stirring occasionally. Add the celery and onion to the pot and sauté for an additional 5 minutes.
Wine & Spices: Add the white wine to the pan, mix well, and sauté for 2 minutes. Then, add the garlic, tamari, sage, thyme, pepper, and nutritional yeast (if using) to the pan. Mix well, and sauté for another 3 to 5 minutes, until most of the liquid has evaporated in the pan.
Combine: Add the vegetable broth, parsley, and toasted bread cubes to the pan and mix well, until the bread is evenly distributed. Transfer the stuffing mixture into the baking dish and use a spatula to spread it out evenly. Cover the baking dish with aluminum foil, or parchment paper followed by foil.
Bake: Bake for 30 minutes covered, then remove the foil and bake for an additional 20 to 25 minutes, until the stop of the stuffing is brown and crisp.
Serve: Remove from the oven and top with additional fresh parsley. Serve warm; stuffing is best served fresh, but leftovers will keep in the fridge for up to 5 days.