Preheat the oven to 300°F. Spread the cubed sourdough bread on a baking sheet. Bake until dry and crispy, about 25-30 minutes, stirring them halfway throug
In a large skillet over medium heat, melt butter and add onions, celery, garlic, and mushrooms. Cook until vegetables are softened and fragrant and mushrooms release their moisture, about 10 minutes.
Transfer the mixture to a greased baking dish, cover with foil, and bake for 30 minutes. Remove the foil and bake for an additional 15 minutes or until the top is golden brown.