Preheat oven to 350°F and line two baking sheets with parchment paper.
Cut sourdough bread into 1-inch cubes and spread it evenly on lined baking sheets. Bake for 20-25 minutes or until bread is completely dry and starts to crisp up and get golden brown. Remove from oven and transfer to a large bowl to cool.
While bread is in the oven, heat butter in a large skillet over medium heat. Add onion, garlic and celery and sauté until onion is fragrant and translucent, about 5-7 minutes.
Add mushrooms and cook for another 6-8 minutes, until mushrooms cook down, start to brown and liquid starts to evaporate.
Add sage, thyme, rosemary, parsley, salt and pepper to the pan and saute for 1 minutes. Add broth and stir to combine. Turn off heat.
Pour liquid mixture over sourdough cubes and stir to combine and fully coat bread cubes.
Transfer mixture into a prepared 9x13 baking dish and bake for 35-45 minutes, until top is golden brown.
Remove from oven and let cool slightly before serving.