Preheat oven to 350 degrees. Lightly spray a 9 x 13-inch baking pan with cooking spray.
Divide cubed bread between two large baking sheets. Bake bread for 10 minutes. Flip, then cook for another 5 to 10 minutes or until edges are crunchy. Set aside.
While bread is cooking, melt butter in a large skillet over medium heat.
Add onions and celery. Cook until the onion edges are starting to turn translucent (about 5 minutes).
Increase heat to medium-high. When skillet is hot, add mushrooms. Cook, stirring frequently, until all liquid released from mushrooms has evaporated and they are starting to brown (about 15 minutes).
Stir in garlic and cook, stirring constantly, until garlic is fragrant (about 30 seconds). Remove skillet from heat, then stir in parsely, sage, and thyme. Season generously to taste with salt and pepper. Let cool slightly before proceeding.
Place bread in a large bowl. Top with mushroom mixture and egg.
Add broth, a little at a time, stirring well after each addition. Add just enough broth so stuffing is moistened, but not soggy.
Transfer stuffing to baking pan, then bake until stuffing is hot and lightly browned on top (about 30 to 40 minutes).
Serve stuffing hot. Enjoy!
Adjusting the oven temperature: if you're cooking something else at a lower or higher temperature, it's fine to cook the stuffing at the same temp. Just keep an eye on the stuffing, and remember to adjust the cooking time up or down, as needed.To prep this recipe ahead of time: cook the veggies and toast the bread, then refrigerate the veggies and store the bread in separate airtight containers until ready to use. On the big day, mix the veggies and bread together, along with the egg and broth. Bake according to directions above, keeping in mind that it may take a little longer since the veggies are cold.