Preheat oven to 350°F (175°C). Spray a large (9 x 13-inch) baking dish with nonstick spray (or coat with butter), then set aside.
Spread cubed bread over 2 large baking sheets. Bake for 10 minutes, then flip. Cook for another 5 to 10 minutes, or until edges are crunchy. Set aside.
While bread cooks, melt 4 tablespoons butter in a large frying pan over medium heat.
When butter is hot, add 3 cups diced celery and 2 cups diced onion, then season with salt and pepper (to taste). Cook until onion starts to turn translucent, stirring frequently (about 7 to 10 minutes).
Add 8 ounces mushrooms, then cook (stirring frequently) until all liquid has evaporated and mushrooms are starting to brown (about 10 to 15 minutes).
Remove pan from heat, then stir in ½ cup chopped parsley, 2 tablespoons chopped sage, and 1 tablespoon chopped rosemary. Season generously with salt and pepper to taste.
Place dried bread cubes in a large bowl. Top with sautéed veggies and 1 large egg.
Mix in 2 to 3 cups broth, a little at a time (stirring well after each addition). Add just enough broth so that the stuffing is moistened, but not soggy. (You may have leftover broth, or you may need more, depending on your bread.)
Transfer stuffing to prepared baking dish, then bake until hot and lightly browned on top (about 35 to 40 minutes).
Serve stuffing hot. Enjoy!
Notes
Bread - Any good quality brioche will work, whether from a bakery or grocery store. I used Nature's Own Perfectly Crafted Brioche, which is easy to find in U.S. stores. Instead of baking the bread cubes, you can let them sit out overnight (uncovered), if you'd prefer.
Mushrooms - I used white mushrooms here, but other varieties (such as brown / baby bella) will also work. Wild mushrooms would be extra delicious! You could also use a full pound (454 grams) for a more mushroom forward stuffing.
Adjusting the Oven Temp - If you're cooking something else at a similar temperature that's a little lower or higher, it's fine to cook the stuffing at the same temp. Just watch the stuffing closely, and adjust the cooking time up or down, as needed.
Make Ahead of Time - To prep the recipe in advance, cook the vegetables and toast the bread, then refrigerate the veggies in an airtight container, and store the bread in a separate airtight container at room temp. 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 slightly longer since the veggies are cold.
More - Check out the info before this recipe card for helpful tips, recipe variations, servings suggestions, and more!