Preheat oven to 275°F and adjust oven racks to upper-middle and lower-middle positions. Butter a 3-qt baking dish; set aside.
Divide the bread evenly between 2 rimmed baking sheets and place on the oven racks. Bake, stirring occasionally, for 40-45 minutes, or until the bread is dried out but not browned. Transfer bread to a large bowl to cool completely.
1½ pounds bread
While the bread bakes, set a large skillet over medium heat and add the sausage. Break the sausage into bite-size pieces during cooking. Cook the sausage, stirring often, for 6-8 minutes, or until brown throughout.
1 pound breakfast sausage
Transfer cooked sausage to a paper towel-lined plate, and discard all except 1 tablespoon of rendered fat from the skillet.
Reduce the heat to medium-low and melt ¾ cup butter in the same skillet. Add the diced onion, fennel, celery, carrots, and apple; mix well. Season with salt and pepper; stir well. Cook, stirring often, until vegetables and apple are tender, about 10-15 minutes. If the vegetables are browning, reduce the heat to low and continue cooking until softened.
¾ cup unsalted butter , 1 large yellow onion, ½ bulb fennel, 2 ribs celery, 2 medium carrots, 1 Granny Smith apple, 1½ teaspoons kosher salt, 1 teaspoon freshly ground black pepper
Deglaze the pan with 1½ cups chicken stock and stir in the parsley, sage, and thyme.
3 cups low-sodium chicken broth, ¼ cup finely chopped parsley, 1 tablespoon finely chopped sage leaves, 1 tablespoon thyme leaves
When the bread has cooled, transfer it to a large bowl. Add the cooked sausage and the vegetable/apple mixture; gently mix.
Increase the oven temperature to 350°F and adjust the oven rack to the middle position.
In a small bowl, whisk together the remaining 1½ cups of broth and eggs. Pour the egg mixture over the bread mixture and fold gently until well combined.
2 large eggs
Let the mixture sit, stirring occasionally, for about 10 minutes, or until the bread absorbs all of the liquid.
Transfer the mixture to the prepared baking dish. Butter (or spray with nonstick spray) one side of a sheet of foil and cover the dish with the foil, greased-side down.
Set the dish on the middle rack and bake, covered, for 35-40 minutes or until very hot to the touch. The internal temperature (tested in the middle) should register 160°F on an instant-read thermometer.
Remove the foil and bake for an additional 30-45 minutes, or until the stuffing is set in the middle and the top is golden brown.
If the stuffing needs more browning after fully cooked, turn the broiler on and cook until the top has browned to your liking. Watch closely.
Transfer to a cooling rack and allow to cool 5-10 minutes before serving.