Heat a skillet over medium heat. Add the sausage and brown thoroughly. Remove and set aside.
In the same pan, melt about 2 Tbsp butter, then add the onions, celery and garlic and cook, stirring frequently, until the vegetables are soft (about 8 to 10 minutes). Remove and set aside.
In the same pan, melt an additional 2 Tbsp butter, then add the mushrooms and increase the heat to high. Cook the mushrooms until soft and all liquid has been evaporated, stirring occasionally. Do this in batches if necessary. Remove and set aside.
Preheat the oven to 350˚F.
In a very large bowl, combine the cubed bread with the cooked sausage and vegetables, eggs, all herbs and pecorino cheese. Season to taste with salt and black pepper. Mix until all ingredients are well-combined. Add most of the broth and mix again. Let the mixture sit for about 15-20 minutes to allow for the bread to absorb the liquid. Then, add additional broth if necessary. The finished mixture should be very moist, but without excess liquid.
Using some of the remaining butter, butter the bottom and sides of a 10” x 8” x 2½” (or similar) baking dish.
Carefully transfer the stuffing mixture to the baking dish and spread it out evenly. Dab the top with the remaining butter, then cover the pan loosely with aluminum foil.
Roast in the 350˚F oven until the stuffing reaches an internal temperature of 165˚F, about 1 ¼ - 1 ½ hours. Remove the aluminum foil about halfway through the cooking process so that the stuffing can form a crust. Buon Appetito!