Preheat the oven to 350°F (175°C). Spread the cubed bread on a baking sheet and toast for 12-15 minutes, or until golden brown and slightly crisp. Set aside to cool.

In a Dutch oven, cook the sage sausage over medium heat, breaking it into small pieces. Cook until browned and fully cooked through (about 5-7 minutes). Remove the sausage from the Dutch oven and set it aside.

In the same Dutch oven, add the diced celery, onion, mushrooms, and minced garlic. Sauté for 8-10 minutes until softened and slightly caramelized. Once the vegetables are softened, stir in the melted butter to coat them evenly. Then, stir in the Better Than Bouillon poultry seasoning, thyme, rosemary, kosher salt, and black pepper. Allow the seasonings to blend into the vegetable mixture for 1-2 minutes.

Gradually pour in the broth. Return the sausage to the Dutch oven and mix thoroughly. Gradually add the toasted bread cubes, stirring as you go. Adjust the broth to ensure the bread is moistened but not soggy. If you prefer a richer texture, whisk in the egg and stir it into the mixture.

Transfer the mixture to a greased 9x13-inch baking dish. If desired, dot the top with 2 tablespoons of patted butter. Cover the dish with foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes until the top is golden brown and crispy.

Remove from the oven, garnish with fresh parsley, and serve warm.