Preheat the oven to 350 degrees F.
Grease a 9x13" dish with cooking spray.
Cut the bread into cubes and place in baking dish.
In a large skillet, melt 4 tablespoons of butter.
Add the leeks, onion, celery, and mushrooms, and cook until softened, stirring frequently, about 10-12 minutes.
Add the vegetables to the bread and give it a little toss to combine.
In a small bowl (or a large 4-cup measuring cup), add the broth and eggs. Whisk to combine.
Pour the mixture evenly over the stuffing.
Slice the remaining 4 tablespoons of butter into thin slices and lay them out evenly over the stuffing. Cover with foil.
Bake covered for 20 minutes and then uncover and bake 15-20 more minutes, until the stuffing is golden brown.
Serve immediately and enjoy!