Set your oven to 300°F and place the rack in the center. Grease a 9x13-inch baking dish with butter, covering the sides and corners well. This keeps the stuffing from sticking and helps the edges brown nicely.
Spread the torn bread in a single layer on a large rimmed baking sheet. Bake for about 45 minutes, stirring every 15 minutes, until the pieces feel dry and firm. Move the bread to a large bowl and let it cool a little.
Set a large skillet over medium-high heat and add the ground sausage. Break it into small crumbles with a spoon as it cooks. After about 5 minutes, it should start to brown and smell amazing.
Add the butter, chopped onion, and diced celery right into the sausage. Stir often and cook for about 5 more minutes. The vegetables should turn soft and the sausage should look fully browned and crumbly.
Pour the sausage mixture over the dried bread. Add the parsley, sage, rosemary, thyme, nutmeg, salt, and pepper. Pour in 1½ cups of chicken broth and toss gently so the bread stays intact.
In a medium bowl, whisk the eggs with the remaining 1 cup of broth. Pour it over the bread and fold gently until everything is evenly coated. Squeeze a small handful; it should hold together without dripping.
Spoon the mixture into the greased dish and spread it evenly. Cover with foil and bake for about 30 minutes, until set. Remove the foil and bake 45 minutes more, until the top is deep golden and crispy.
Let the dish sit for about 10 minutes before scooping. This rest helps it firm up so each portion holds its shape. Serve it hot straight from the baking dish.