Cut bread into cubes totaling 10 cups and spread it on a cookie sheet. Let it sit overnight to dry out or pop it in a 300° F (148° C) oven for about 20 minutes until the bread is dry and crispy. (You want it to just dry out, not brown!)
Make the stuffing
In a large saucepan, saute a finely diced onion in 1/4 cup of butter (or olive oil) over low heat until the onion becomes translucent. This will take 5-7 minutes.
Add the chopped celery, carrot, and mushroom to the onions and sprinkle with 1 tablespoon of poultry seasoning and 1/4 tsp of salt. Stir well and saute the veggies for about 5 more minutes until they are soft.
Toss in the 10 cups of cubed dry bread and stir well to coat the bread cubes in the veggies and seasoning.
Pour the mixture into an oiled 8 x 8-inch baking dish and drizzle with 2 cups of vegan broth, coating the bread, and give it all a little stir. (At this point you can place the stuffing covered in the fridge for up to 2 days before baking.
Bake the stuffing
Bake the stuffing uncovered at 375° F (190° C) for 35 minutes until crispy and slightly brown on the top. (For stuffing with a crispier top, drizzle 2 tablespoons of olive oil over the top of the stuffing before baking.)
Notes
How to make the stuffing ahead of timeIf you want to prep your Thanksgiving feast ahead of time follow steps 1-5 in the instructions, then cover and refrigerate the stuffing in a covered baking dish for up to 2 days. When you are ready to bake, preheat your oven to 375° F (148° C) and bake for about 40 minutes. (It will need about 5 extra minutes in the oven since it will be cold.)