Preheat oven to 350°F (175°C). Spray a 9 x 13-inch baking pan with nonstick spray (or lightly oil).
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 cooks, melt butter in a large skillet over medium heat. Add onions and celery; season with salt and pepper. Cook until starting to turn translucent, about 7 to 10 minutes.
Add mushrooms and cook, stirring occasionally, until any liquid is evaporated and mushrooms are starting to brown (about 10 to 15 minutes).
Remove from heat. Stir in parsley, sage, and thyme, then season generously to taste with additional salt and pepper.
Place dried bread cubes in a large bowl. Top with veggies 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 dish, then bake until hot and lightly browned on top (about 35 to 40 minutes).
Serve hot. Enjoy!
Notes
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 it, 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.
Check out info before this recipe for lots of tips, tricks, and ideas, such as recipe variations.