One - three days in advance: Cut or tear the bread into cubes, spread on a baking sheet or two and let dry (uncovered) on the counter until completely dry like croutons. I like a combination of crusty ciabatta bread and French bread baguette.
The day of: Preheat the oven to 375°F. Butter a 9x11" baking dish. Mix up sage and onion seasoning. Transfer the dry bread cubes to a large mixing bowl.
Melt the non-dairy butter in a large skillet over medium-high heat. Then add the onions, celery, garlic, salt, and pepper and cook until softened (~8-10 minutes).
Stir in 2 tablespoons sage and onion seasoning. Add 2 cups veggie stock and let it come up to bubbling. Taste and adjust seasoning to your preference (more sage and onion seasoning, salt, pepper, etc). Pour over the bread cubes and stir/toss to combine completely.
Slowly ladle in more vegetable broth tossing together until the stuffing has the moisture level you want (wet - no dry patches). The amount of broth you'l need will depend on how dense the bread is and how moist you like your dressing. Once you’ve got the moisture right, give it a taste, adjust seasoning if needed then add any chopped fresh herbs.
Transfer to a large buttered baking dish, dot the top of the stuffing with butter then sprinkle the mushrooms and additional fresh herbs over top. Bake uncovered for 25-35 minutes or until the surface is golden and crispy. Add more broth as needed and add a tinfoil tent if the stuffing is getting too dark.