Dry the bread overnight- slice or tear the bread into 1 inch cubes of relatively equal size. Spread the bread cubes out on a sheet pan and cover with a clean kitchen towel. Allow to sit at room temperature for at least 24 hours.
Day 2- preheat the oven to 400°F.
Sauté the mushrooms- add 2 tablespoons of vegan butter to a large frying pan heated to medium heat. Once the butter is melted, add the mushrooms. Cook the mushrooms, stirring occasionally, for 10-12 minutes, until the mushrooms develop a deep brown color. Remove from heat and transfer to a clean bowl.
Sauté the leeks- add 2 more tablespoons of vegan butter to the frying pan. Once the butter is melted, add the leeks and celery. Cook for 3-4 minutes, then add the chopped rosemary and thyme leaves, and the minced garlic. Cook for a total of 5-7 minutes, until leeks and celery are softened.
Mix up the stuffing- in a large bowl, add the dried breadcrumbs, sautéed mushrooms, leek/celery/herb mixture and half a cup of chopped walnuts. Mix together the vegetable broth and salt in a measuring cup or bowl, then pour half of this mixture over the stuffing ingredients. Mix the stuffing up well.
Transfer to the baking dish- grease a 9 x 13 inch baking dish with spray oil or vegan butter. Scrape the stuffing into the baking dish, then pour the rest of the vegetable stock evenly over top. Press the stuffing firmly down into the pan.
Bake- cover the baking dish with a reusable silicone lid or aluminum foil. Bake in the oven for 30 minutes. Remove the cover and return to the oven. Bake for an additional 20-25 minutes, until the top of the stuffing is golden brown and slightly crispy to the touch.
Serve- once stuffing is baked through, allow it to sit for 10 minutes before serving.