Spread the bread cubes evenly over two baking sheets lined with parchment paper or aluminum foil. Bake the cubes in oven until completely dried and beginning to harden and turn golden, about 15-20 minutes.
Melt butter in a large skillet over medium-high heat. Add the chopped mushrooms, 1 tsp. salt and 1/2 tsp. pepper and saute until mushrooms are golden brown, stirring occasionally, about 10 minutes.
Once golden, add celery, onion and thyme. Cook, stirring occasionally, until the celery and onion have softened, about 8-10 minutes.
Add sage once vegetables are softened. Add chicken broth and stir mixture to combine. Season with salt and pepper to taste.
Carefully pour the bread mixture into a 9x13 or similar baking dish. Take care not to smash the stuffing into the dish, just lay it gently so a not to mush the bread too much.